Can't figure out why capture group is not output by substitution regex

[nysus]

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";

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

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

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.

[hippo]

I'm not sure why I have to do that.

You have to do it because your second (inner) s/// resets the capture groups.

[nysus]

Ah, of course. Thanks.

[Eily]

I can fix this by assigning $2 go a new scalar variable first and then using the new variable.

Or simply $here =~ s/(^(#+)\s.*?\g2)/$1 =~ s|\n||rg/mse;

[nysus]

Yes, much simpler. Thanks.