use strict;
use warnings;

use Getopt::Long;

my $debug = "";

GetOptions( 'd=i' => \$debug);

if ($debug eq "") {
  # -d flag was not used
}
else {
  if (($debug < 1) && ($debug > 4)) {
    # -d flag used, but value is not acceptable
    my $msg = "The value of '$debug' is not valid for -d option.\n";
    $msg = "The -d option only accepts the values between 1 and 4.\n"
    die "$msg";
  }
}