Filter::signatures - very simplistic signatures for Perl < 5.20


    use Filter::signatures;
    no warnings 'experimental::signatures'; # does not raise an error
    use feature 'signatures'; # this now works on <5.20 as well

    sub hello( $name ) {
        print "Hello $name\n";


    sub hello2( $name="world" ) {
        print "Hello $name\n";
    hello2(); # Hello world


This module implements a backwards compatibility shim for formal Perl subroutine signatures that were introduced to the Perl core with Perl 5.20.


The technique used is a very simplistic transform to allow for using very simplistic named formal arguments in subroutine declarations. This module does not implement warning if more or fewer parameters than expected are passed in.

The module also implements default values for unnamed parameters by splitting the formal parameters on /,/ and assigning the values if @_ contains fewer elements than expected. Function calls as default values may work by accident. Commas within default values happen to work due to the design of Filter::Simple, which removes them for the application of this filter.

Note that this module inherits all the bugs of Filter::Simple and potentially adds some of its own. Most notable is that Filter::Simple sometimes will misinterpret the division operator / as a leading character to starting a regex match:

    my $wait_time = $needed / $supply;

This will manifest itself through syntax errors appearing where everything seems in order. The hotfix is to add a comment to the code that "closes" the misinterpreted regular expression:

    my $wait_time = $needed / $supply; # / for Filter::Simple

A better hotfix is to upgrade to Perl 5.20 or higher and use the native signatures support there. No other code change is needed, as this module will disable its functionality when it is run on a Perl supporting signatures.

Parentheses in default expresisons

Ancient versions of Perl before version 5.10 do not have recursive regular expressions. These will not be able to properly handle statements such as

    sub foo ($timestamp = time()) {

The hotfix is to rewrite these function signatures to not use parentheses. The better approach is to upgrade to Perl 5.20 or higher.

Regular expression matches in default expressions

To keep the argument parser simple, the parsing of regular expressions has been omitted. For Perl below 5.10, you cannot use regular expressions as default expressions. For higher Perl versions, this means that parentheses, curly braces and commas need to be explicitly escaped with a backslash when used as default expressions:

    sub foo( $x = /,/ ) { # WRONG!
    sub foo( $x = /\,/ ) { # GOOD!

    sub foo( $x = /[(]/ ) { # WRONG!
    sub foo( $x = /[\(]/ ) { # GOOD!

The hotfix is to rewrite these default expressions with explicitly quoted commas, parentheses and curly braces. The better approach is to upgrade to Perl 5.20 or higher.

Subroutine attributes

Subroutine attributes are currently not supported at all.

Line Numbers

Due to a peculiarity of how Filter::Simple treats here documents in some versions, line numbers may get out of sync if you use here documents.

If you spread your formal signatures across multiple lines, the line numbers may also go out of sync with the original document.


Filter::Simple does not trigger when using code such as

  eval <<'PERL';
      use Filter::signatures;
      use feature 'signatures';

      sub foo (...) {

So, creating subroutines with signatures from strings won't work with this module. The workaround is to upgrade to Perl 5.20 or higher.


The generated code does not deparse identically to the code generated on a Perl with native support for signatures.


If you want to force the use of this module even under versions of Perl that have native support for signatures, set $ENV{FORCE_FILTER_SIGNATURES} to a true value before the module is imported.


If you have a source file that was written for use with signatures and you cannot modify that source file, you can run it as follows:

  perl -Mlib=some/directory -MFilter::signatures=global

This is intended as a quick-fix solution and is not very robust. If your script modifies @INC, the filtering may not get a chance to modify the source code of the loaded module.

This currently does not play well with (other) hooks in @INC as it only handles hooks that return a filehandle. Implementations for the rest are welcome.


"Signatures" in perlsub

App::sigfix, which transforms your source code directly between the different notations without employing a source filter

signatures - a module that doesn't use a source filter but optree modification instead

Sub::Signatures - uses signatures to dispatch to different subroutines based on which subroutine matches the signature

Method::Signatures - this module implements subroutine signatures closer to Perl 6, but requires PPI and Devel::Declare

Function::Parameters - adds two new keywords for declaring subroutines and parses their signatures. It supports more features than core Perl, closer to Perl 6, but requires a C compiler and Perl 5.14+.


The public repository of this module is


The public support forum of this module is


Please report bugs in this module via the RT CPAN bug queue at or via mail to


Max Maischein


Copyright 2015-2020 by Max Maischein


This module is released under the same terms as Perl itself.