I would probably write your script something like the following, avoiding the problem of how many passwords to generate.

use strict;
use warnings;

my $users = "new.users";
open(my $in, '<', $users) or die "Cannot open $users: $!";

my $tmp = $users . '.tmp';
open(my $out, '>', $tmp) or die "Cannot open $tmp: $!";

foreach (<$in>) {
    if(/^userPassword: xyz456$/) {
        my $pw = generatePassword(10);
        s/^userPassword: xyz456/userPassword: $pw/;
    }
    print $out "$_";
}

close($out);
close($in);

unlink($users) or die "unlink($users): $!";
rename($tmp, $users) or die "rename($tmp, $users): $!";

exit(0);



sub generatePassword {
    my $length = shift;
    my $password = "";
    my $possible = 'abcdefghijkmnpqrstuvwxyz23456789ABCDEFGHJKLMNPQRST
+UVWXYZ';
    while (length($password) < $length) {
    $password .= substr($possible, (int(rand(length($possible)))), 1);
    }
    return $password
}
[download]