The special value 00 will cause Perl to «slurp» files in paragraph mode. Any value 0400 or above will cause Perl to slurp files whole, but by convention the value 0777 is the one normally used for this purpose.

You can also specify the separator character using hexadecimal notation: -0xHHH. where the H are valid hexadecimal digits. Unlike the octal form, this one may be used to specify any Unicode character, even those beyond 0xFF. So if you really want a record separator of 0777, specify it as -0x1FF. (This means that you cannot use the -x option with a directory name that consists of hexadecimal digits, or else Perl will think you have specified a hex number to -0.)

is equivalent to

An alternate delimiter may be specified using -F.

As of 5.8.1, the -C can be followed either by a number or a list of option letters. The letters, their numeric values, and effects are as follows; listing the letters is equal to summing the numbers.

For example, -COE and -C6 will both turn on UTF-8-ness on both STDOUT and STDERR. Repeating letters is just redundant, not cumulative nor toggling.

The io options mean that any subsequent open() (or similar I/O operations) in the current file scope will have the :utf8 PerlIO layer implicitly applied to them, in other words, UTF-8 is expected from any input stream, and UTF-8 is produced to any output stream. This is just the default, with explicit layers in open() and with binmode() one can manipulate streams as usual.

-C on its own (not followed by any number or option list), or the empty string «» for the PERL_UNICODE environment variable, has the same effect as -CSDL. In other words, the standard I/O handles and the default open() layer are UTF-8-fied but only if the locale environment variables indicate a UTF-8 locale. This behaviour follows the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0. (See UTF-8 no longer default under UTF-8 locales in perl581delta.)

You can use -C0 (or «0» for PERL_UNICODE ) to explicitly disable all the above Unicode features.

The read-only magic variable $ reflects the numeric value of this setting. This variable is set during Perl startup and is thereafter read-only. If you want runtime effects, use the three-arg open(), the two-arg binmode(), and the open pragma.

As an alternative, specify a number instead of list of letters (e.g., -D14 is equivalent to -Dtls):

All these flags require -DDEBUGGING when you compile the Perl executable (but see :opd in Devel::Peek or ‘debug‘ mode in re which may change this). See the INSTALL file in the Perl source distribution for how to do this. This flag is automatically set if you include -g option when Configure asks you about optimizer/debugger flags.

If you’re just trying to get a print out of each line of Perl code as it executes, the way that sh -x provides for shell scripts, you can’t use Perl’s -D switch. Instead do this:

Perl can be built so that it by default will try to execute $Config/sitecustomize.pl at startup (in a BEGIN block). This is a hook that allows the sysadmin to customize how Perl behaves. It can for instance be used to add entries to the @INC array to make Perl find modules in non-standard locations.

Perl actually inserts the following code:

Since it is an actual do (not a require), sitecustomize.pl doesn’t need to return a true value. The code is run in package main, in its own lexical scope. However, if the script dies, [email protected] will not be set.

The value of $Config is also determined in C code and not read from Config.pm, which is not loaded.

If no extension is supplied, and your system supports it, the original file is kept open without a name while the output is redirected to a new file with the original filename. When perl exits, cleanly or not, the original file is unlinked.

If the extension doesn’t contain a *, then it is appended to the end of the current filename as a suffix. If the extension does contain one or more * characters, then each * is replaced with the current filename. In Perl terms, you could think of this as:

This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix:

Or even to place backup copies of the original files into another directory (provided the directory already exists):

These sets of one-liners are equivalent:

From the shell, saying

is the same as using the program:

which is equivalent to

except that the -i form doesn’t need to compare $ARGV to $oldargv to know when the filename has changed. It does, however, use ARGVOUT for the selected file handle. Note that STDOUT is restored as the default output file handle after the loop.

As shown above, Perl creates the backup file whether or not any output is actually changed. So this is just a fancy way to copy files:

You can use eof without parentheses to locate the end of each input file, in case you want to append to each file, or reset line numbering.

If, for a given file, Perl is unable to create the backup file as specified in the extension then it will skip that file and continue on with the next one (if it exists).

You cannot use -i to create directories or to strip extensions from files.

Perl does not expand

in filenames, which is good, since some folks use it for their backup files:

Note that because -i renames or deletes the original file before creating a new file of the same name, Unix-style soft and hard links will not be preserved.

Finally, the -i switch does not impede execution when no files are given on the command line. In this case, no backup is made (the original file cannot, of course, be determined) and processing proceeds from STDIN to STDOUT as might be expected.

Note that the assignment $\ = $/ is done when the switch is processed, so the input record separator can be different than the output record separator if the -l switch is followed by a -0 switch:

This sets $\ to newline and then sets $/ to the null character.

-Mmodule executes use module ; before executing your program. You can use quotes to add extra code after the module name, e.g., ‘-MMODULE qw(foo bar)’.

If the first character after the -M or -m is a dash (—) then the ‘use‘ is replaced with ‘no‘.

You can also say -mMODULE=foo,bar or -MMODULE=foo,bar as a shortcut for ‘-MMODULE qw(foo bar)’. This avoids the need to use quotes when importing symbols. The actual code generated by -MMODULE=foo,bar is use module split(/,/,q<foo,bar>). Note that the = form removes the distinction between -m and -M.

A consequence of this is that -MMODULE=number never does a version check, unless MODULE::import() itself is set up to do a version check, which could happen for example if MODULE inherits from Exporter.

Note that the lines are not printed by default. See -p to have lines printed. If a file named by an argument cannot be opened for some reason, Perl warns you about it and moves on to the next file.

Also, note that <> passes command line arguments to open, which doesn’t necessarily interpret them as file names.

Here is an efficient way to delete all files that haven’t been modified for at least a week:

This is faster than using the -exec switch of find because you don’t have to start a process on every filename found. It does suffer from the bug of mishandling newlines in pathnames, which you can fix if you follow the example under -0.

BEGIN and END blocks may be used to capture control before or after the implicit program loop, just as in awk.

If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal. To suppress printing use the -n switch. A -p overrides a -n switch.

BEGIN and END blocks may be used to capture control before or after the implicit loop, just as in awk.

Do note that a switch like —help creates the variable $ , which is not compliant with use strict «refs». Also, when using this option on a script with warnings enabled you may get a lot of spurious «used only once» warnings.

On some platforms, this also makes Perl append suffixes to the filename while searching for it. For example, on Win32 platforms, the «.bat» and «.cmd» suffixes are appended if a lookup for the original name fails, and if the name does not already end in one of those suffixes. If your Perl was compiled with DEBUGGING turned on, using the -Dp switch to Perl shows how the search progresses.

Typically this is used to emulate #! startup on platforms that don’t support #!. It’s also convenient when debugging a script that uses #!, and is thus normally found by the shell’s $PATH search mechanism.

This example works on many platforms that have a shell compatible with Bourne shell:

The system ignores the first line and feeds the program to /bin/sh, which proceeds to try to execute the Perl program as a shell script. The shell executes the second line as a normal shell command, and thus starts up the Perl interpreter. On some systems $0 doesn’t always contain the full pathname, so the -S tells Perl to search for the program if necessary. After Perl locates the program, it parses the lines and ignores them because the variable $running_under_some_shell is never true. If the program will be interpreted by csh, you will need to replace $<1+"[email protected]"> with $*, even though that doesn’t understand embedded spaces (and such) in the argument list. To start up sh rather than csh, some systems may have to replace the #! line with a line containing just a colon, which will be politely ignored by Perl. Other systems can’t control that, and need a totally devious construct that will work under any of csh, sh, or perl, such as the following:

If the filename supplied contains directory separators (and so is an absolute or relative pathname), and if that file is not found, platforms that append file extensions will do so and try to look for the file with those extensions added, one by one.

On DOS-like platforms, if the program does not contain directory separators, it will first be searched for in the current directory before being searched for on the PATH. On Unix platforms, the program will be searched for strictly on the PATH.

Note: This is not a substitute for -T! This is meant to be used only as a temporary development aid while securing legacy code: for real production code and for new secure code written from scratch, always use the real -T.

Additionally, extra colons can be used to control formatting. A trailing colon suppresses the linefeed and terminator «;«, allowing you to embed queries into shell commands. (mnemonic: PATH separator «:«.)

A leading colon removes the «name=» part of the response, this allows you to map to the name you need. (mnemonic: empty label)

Leading and trailing colons can be used together if you need positional parameter values without the names. Note that in the case below, the PERL_API parameters are returned in alphabetical order.

This switch really just enables the global $^W variable; normally, the lexically scoped use warnings pragma is preferred. You can disable or promote into fatal errors specific warnings using __WARN__ hooks, as described in perlvar and warn. See also perldiag and perltrap. A fine-grained warning facility is also available if you want to manipulate entire classes of warnings.

All references to line numbers by the program (warnings, errors, . ) treat the #! line as the first line. Thus a warning on the 2nd line of the program, which is on the 100th line in the file, is reported as line 2, not as line 100. This can be overridden using the #line directive.

If a directory name is specified, Perl switches to that directory before running the program. The -x switch controls only the disposal of leading garbage. The program must be terminated with __END__ if there is trailing garbage to be ignored; the program can process any or all of the trailing garbage via the DATA file handle if desired.

The directory, if specified, must appear immediately following the -x with no intervening whitespace.

It is conventional to start layer names with a colon (for example, :perlio) to emphasize their similarity to variable «attributes». But the code that parses layer specification strings, which is also used to decode the PERLIO environment variable, treats the colon as a separator.

An unset or empty PERLIO is equivalent to the default set of layers for your platform; for example, :unix:perlio on Unix-like systems and :unix:crlf on Windows and other DOS-like systems.

The list becomes the default for all Perl’s IO. Consequently only built-in layers can appear in this list, as external layers (such as :encoding() ) need IO to load them. See open pragma for how to add external encodings as defaults.

Layers that make sense to include in the PERLIO environment variable are briefly summarized below. For more details see PerlIO.

The default set of layers should give acceptable results on all platforms.

For Unix platforms that are the equivalent of «unix perlio» or «stdio«. Configure is set up to prefer the «stdio» implementation if the system’s library provides for fast access to the buffer; otherwise, it uses the «unix perlio» implementation.

On Win32 the default in this release (5.14) is «unix crlf«. Win32’s «stdio» has a number of bugs/mis-features for Perl IO which are somewhat depending on the version and vendor of the C compiler. Using our own crlf layer as the buffer avoids those issues and makes things more uniform. The crlf layer provides CRLF conversion and buffering.

The current release (5.14 as of this writing) uses unix as the bottom layer on Win32, and so still uses the C compiler’s numeric file descriptor routines. There is an experimental native win32 layer, which is expected to be enhanced and should eventually become the default under Win32.

The PERLIO environment variable is completely ignored when Perl is run in taint mode.

and under Win32, the approximately equivalent:

This functionality is disabled for setuid scripts and for scripts run with -T.

The PERLLIB environment variable is completely ignored when Perl is run in taint mode.

The PERL5DB environment variable is only used when Perl is started with a bare -d switch.

Note that Perl doesn’t use COMSPEC for this purpose because COMSPEC has a high degree of variability among users, leading to portability concerns. Besides, Perl can use a shell that may not be fit for interactive use, and setting COMSPEC to such a shell may interfere with the proper functioning of other programs (which usually look in COMSPEC to find a shell fit for interactive use).

Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked when running external commands. It is recommended that you explicitly set (or delete) $ENV when running in taint mode under Windows.

Setting this environment variable to 1 means that Perl will use the first suitable LSP enumerated in the catalog, which keeps McAfee Guardian happy, and in that particular case Perl still works too because McAfee Guardian’s LSP actually plays other games which allow applications requiring IFS compatibility to work.

If set, this dumps out memory statistics after execution. If set to an integer greater than one, also dumps out memory statistics after compilation.

If the option is provided, and PERL_PERTURB_KEYS is NOT set, then a value of ‘0‘ implies PERL_PERTURB_KEYS=0 and any other value implies PERL_PERTURB_KEYS=2.

PLEASE NOTE: The hash seed is sensitive information. Hashes are randomized to protect against local and remote attacks against Perl code. By manually setting a seed, this protection may be partially or completely lost.

When set to «1» or «RANDOM» then traversing keys will be randomized. Every time a hash is inserted into the key order will change in a random fashion. The order may not be repeatable in a following program run even if the PERL_HASH_SEED has been specified. This is the default mode for perl.

When set to «2» or «DETERMINISTIC» then inserting keys into a hash will cause the key order to change, but in a way that is repeatable from program run to program run.

NOTE: Use of this option is considered insecure, and is intended only for debugging non-deterministic behavior in Perl’s hash function. Do not use it in production.

Note that any information about the hash function, especially the hash seed is sensitive information: by knowing it, one can craft a denial-of-service attack against Perl code, even remotely. Do not disclose the hash seed to people who don’t need to know it. See also hash_seed() and key_traversal_mask() in Hash::Util.

An example output might be: