hakkum:

Just a few minor notes:

• You're processing a command-line argument inside of random_password. That gives you a hidden data path that can be difficult to work with in a larger program.
• Comments like # define variables are useless, as it's quite obvious that you're doing just that. If the code is obvious, you don't need the comment. If the code isn't obvious, then usually it's better to make it obvious rather than comment it. For example:
  

  # default password length
  $pass_len = 14;
  [download]

  Would be better as:
  

  $dflt_pass_len = 14;
  [download]

• Declaring your variables at the point of use is better practice.
• I prefer using return expression rather than expression to return a value to the caller.

Here's a slight update of the random_password subroutine for demonstration:

sub random_password {
    my ($pass_len) = @_;

    my @chars_in_pass = character_pool();

    my $password='';
    for (1..$pass_len) {
        my $random_number = int rand @chars_in_pass;
        $password     .= $chars_pool[$random_number];
    }

    return $password;
}
[download]

I don't mean to indicate that your program is bad in any way. Overall, it's an impressive first program.

...roboticus

When your only tool is a hammer, all problems look like your thumb.