system("$useradd -c \"$fullname\" ...");

##</code><code>##

system( 
    $useradd, 
    qq|-c "fullname"|,
    ...

##</code><code>##

    -c "value-of-$fullname"

##</code><code>##

system ($useradd,
    '-c', $fullname,
    '-d', "/home/sites/site$site_count/users/$username",
    '-g', "site$site_count",
    '-G', "site-adm$site_count",
    '-p', $password,
    '-s', '/bin/false',
    '-u', $uid, $username);

##</code><code>##

system('cat -n /etc/group');
system('cat', '-n /etc/group');
system('cat', '-n', '/etc/group');