hex-numbers.pl

#!/usr/bin/env perl
use strict;
use warnings;

my $hexnum = shift || '0xdeadbeaf';
$hexnum =~ /0x([0-9a-fA-F]+)/ or die 'not hex number';
my $body = $1;
my $result = 0;
my $cnt = 0;
for (reverse split //, $body) {
    if (/[a-fA-f]/) {
        tr/a-fA-F/0-50-5/;
        $_ += 10;
    }
    $result += $_ * (16 ** $cnt++);
}
warn $result;