I'm using a non-capturing grouping which seems to be capturing.
I expect the code (?:|([^]]+))? to not capture the pipe character, but it does.
Code:

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
use URI;

my $text = <<END;
this is some plain text
here is a link: [link://forum] and here is another: 
[mentors://pil_description|Partnerships in Learning]
END

$text =~ s!
          \[
            (\w+)
            ://
            ([\w-]+)
            (?:|([^]]+))?
          \]
          !print("'$1' '$2' '$3'\n")!gsex;
[download]

Output:

'link' 'forum' ''
'mentors' 'pil_description' '|Partnerships in Learning'
Use of uninitialized value in concatenation (.) or string at test.pl l
+ine 13.
[download]

Are any of my modifiers (/gsex) affecting the capturing, or is there anything else I'm missing?