substr($string, 0, $-[0])

##</code><code>##

#!/usr/bin/perl -wT
use strict;

while (<DATA>) {
  chomp;
  $_ = substr($_, 0, $-[0]) if /#/;
  print "$_\n" if $_;
}

__DATA__
### Hello
99:88:77

100:11# This is a comment
abc:def ### Comments also
999