A couple of quick comments:

• Your pattern match does not account for the fact that the Listen directive may also include a listening port, ie: 1.2.3.4:8080
• You don't tell us anything about your gethost() method. Are you sure that it always returns a meaningful value?

Here is a bit of working (tested) code that emulates what you are trying to do (except for poulating the hash), and does hostname lookups using gethostbyaddr. Perhaps this will help:

#!/usr/bin/perl -w
use strict;
use Socket;

while (<DATA>) {
    if (/^Listen/) {
        chomp;
        my ($ip) = $_ =~ m/^Listen\s([\d\.]+):?/;
        my $host = gethostname($ip) || "unknown";
        print "IP:$ip HOST:$host\n";
    }
}

sub gethostname {
    my $ip = shift;
    my $iaddr = inet_aton($ip);
    return gethostbyaddr($iaddr, AF_INET);
}

__DATA__
Listen 1.2.3.4
Listen 5.6.7.8:9735
rubbish
Listen 209.131.36.158
Listen 11.12.1.4:8080
more rubbish
[download]

Output:

IP:1.2.3.4 HOST:unknown
IP:5.6.7.8 HOST:unknown
IP:209.131.36.158 HOST:f1.www.vip.sp1.yahoo.com
IP:11.12.1.4 HOST:unknown
[download]

To check the contents of your hash, you might consider using Data::Dumper.

Cheers,
Darren