Yeah, what pfaut said. As an additional note, your chomp removes the end line, so your test would always pass. If you really want to catch bad input, you could try:

use strict; 
my $port = shift; 

our $stopit = 0; 
$SIG{INT} = sub { $stopit++; }; 

if (!$port or $port =~ /\D/) { 
    print "usage: program port_num\n"; 
} else { 
    while (!$stopit) { 
        print system("netstat -an | grep $port"), "\n"; 
        sleep 10; 
    } 
}
[download]

or whatever variant will get you what you want.