#! /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";