See perlretut and perlre.

use strict;
use warnings;

while (<DATA>) {
    chomp;
    next unless /description\s+ ([^\s]+)/x;
    
    print "$1\n";
}

__DATA__
description c3560-a68.mud g6/20-g7/2
description c3560-a68.mud :::Leaf:::
description c3560-a68.mud blah blah
[download]

Prints:

c3560-a68.mud
c3560-a68.mud
c3560-a68.mud
[download]