Another way to do it is to take advantage of prematch ($`): 
use strict;
while (<DATA>) {
   chomp;
   $_ = $` if /#/;
   print "$_\n" if $_;
}

__DATA__
### Hello
99:88:77

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

[download]

--Jim

Update: shortenned conditional after chomp;

In reply to Re: Support for hash comments on a line by jlongino
in thread Support for hash comments on a line by c

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