I got this:

#! /usr/bin/env perl

use strict;
use warnings;

my $here = <<HERE;
before text
more text
### hjh jh ja jhjasdhf jh'j
asdk kas dkjakdjf kasjd fkjasdkfj
kajsdkjf kasjdf end ###
after headline
after text
HERE

$here =~ s/(^(#+)\s)(.*?)\g2/$2 . ' ' . $3 =~ s|\n||rg . $2/mse;
print $here . "\n";
[download]

I expect this to be output:

before text
more text
### hjh jh ja jhjasdhf jh'jasdk kas dkjakdjf kasjd fkjasdkfjkajsdkjf k
+asjdf end ###
after headline
after text
[download]

Output I get:

Use of uninitialized value $2 in concatenation (.) or string at /Users
+/me/perl/workshop/regex/tut.pl line 16.
before text
more text
### hjh jh ja jhjasdhf jh'jasdk kas dkjakdjf kasjd fkjasdkfjkajsdkjf k
+asjdf end
after headline
after text
[download]

I can fix this by assigning $2 go a new scalar variable first and then using the new variable. But I'm not sure why I have to do that.