I recommend you:

use warnings;
use strict;
[download]

This will tell you exactly what the problem is. On my system I get this:

Can't exec "umask": No such file or directory at umask.pl line 4.

$ which umask
[download]

Indicates that it's not an actual command you can call. Why aren't you just using the umask perl function?

umask
Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.