#!/usr/bin/perl

# https://perlmonks.org/?node_id=1221039

use strict;
use warnings;

my $someTTstring = <<END;
before
[% # this is a comment to the end of line
   foo = 'bar'
%]

<p>bw, bliako</p>

[%# placing the '#' immediately inside the directive
    tag comments out the entire directive
%]
[% outside %]
[%# placing the '#' immediately inside the directive
    tag comments out the entire directive
    [% inside %]
%]
after
END

print stripcomments($someTTstring);

sub stripcomments
  {
  @_ and local $_ = shift;
  my $answer = '';
  $answer .=
    /\G\[\%#/gc ? stripcomments() x 0 :
    /\G\[\%/gc ? '[%' . stripcomments() =~ s/#.*//gr . '%]' :
    /\G\%\]/gc ? return $answer :
    /\G./gcs ? $& :
    return $answer while 1;
  }
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Like this?

In my attempts I have omitted @_ and and was getting Use of uninitialized value $_, but forgotten about when you call it later on without any params. So, that works for multiline comments but not single line. I have put the following test input which I presume is valid Template::Toolkit comment syntax with the exception of nested comments which I am not sure. I will go the Parser way haukex posted. thanks

 [%# comment nust be removed 1 %]
 [% # comment nust be removed 2 
        must stay 0
%]
 [% # comment nust be removed 3
        must stay 1
 %]
 [% # comment nust be removed 4
        must stay 2
        # comment nust be removed 5
        [%# nested comment (not sure if allowed or not) 1 %]
  %]
 [%# 
        comment nust be removed 6
        comment nust be removed 7
        comment nust be removed 8

        [%# nested comment (not sure if allowed or not) 2 %]
  %]
 [% #
        comment nust be removed 9

        [% comment nust be removed 10 %]
        [% # nested comment (not sure if allowed or not) 2 %]
  %]

[% must stay 3 %]
[% must stay 4
        must stay 5
        # comment nust be removed 11
%]

 [% # comment nust be removed 12
        must stay 6
%]
[download]

Both

 comment nust be removed 9
[download]

and

comment nust be removed 10
[download]

are incorrect, they should both stay under the rules you posted.