Or maybe in more Perlish lingo:

print "Password: ";
chomp(my $passwd = <STDIN>);

system(
    '/usr/sbin/useradd',
    -u => $max + 1,
    -s => $shell,
    -p => crypt($passwd, time),
    -g => 100,
    '-m',
    $username,
);
[download]

:-) Btw, I recommend removing the setuid bit from useradd ASAP. Otherwise, anyone can do something like

$ /usr/sbin/useradd -u 0 -g 0 -s /bin/bash -p crypted_passwd_here -m root2

which is a classic escalation of privileges attack. Instead, you want to look into sudo. Your /etc/sudoers should have a line like this:

newuser ALL = (root) NOPASSWD: /usr/sbin/useradd

and then your code changes to

system(
    '/usr/bin/sudo',
    '/usr/sbin/useradd',
    -u => $max + 1,
    -s => $shell,
    -p => crypt($passwd, time),
    -g => 100,
    '-m',
    $username,
);
[download]

That way only newuser, whom only root has control of, may add new users.

Makeshifts last the longest.