#!/usr/bin/perl

use warnings;
use strict;
use Getopt::Long;
use 5.010; # Just to use say()


my ($mean, $sum);
my $result = GetOptions ("mean"  => \$mean);
my @nums = @ARGV;

foreach (@nums) {
    say;

    # Only calculate the mean if flag exists
    if ($mean) {
        $sum += $_;
    }
}

say $sum/@nums if $mean;

__END__
$ perl mean.pl 1 3 2
1
3
2
$ perl mean.pl -m 1 3 2
1
3
2
2
$ perl mean.pl -mean 1 3 2
1
3
2
2