After reading chapter 5 of japhy's book, you'll be able to rewrite prematch as:

substr($string, 0, $-[0])
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For example:

#!/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
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-Blake