#!/usr/bin/perl

use strict;
use Linux::usermod;

my %users =  Linux::usermod::users();

print join "\n", keys %users

__END__

# STDOUT:
backup
bin
cv
daemon
games
gnats
helpdesk
identd
irc
list
lp
mail
man
mysql
netsaint
news
nobody
operator
popa3d
postfix
postgres
proxy
reni
root
snort
spong
sshd
su
sync
sys
uucp
vidul
vis
www-data

# all those looks like system user accounts to me :)