#!/usr/bin/perl

use strict;
use warnings;

while (my $url = <DATA>) {
    chomp($url);
    if ($url !~ m/tag=mobilfutalk-21/) {
        $url .= ($url =~ /\?/ ? '&' : '?') . 'tag=mobilfutalk-21'
    }
    print "$url\n";
}

__DATA__
http://www.amazon.de/preis/dp/B00CL5X86K/?_encoding=UTF8&camp=1638&creative=6742&linkCode=ur2&site-redirect=de&tag=mobilfutalk-21
http://www.amazon.de/preis/B00CL5X86K/
http://www.amazon.de/preis/B00CL5X86K
http://www.amazon.de/preis/dp/B00CL5X86K/?camp=1638&creative=6742&linkCode=ur2&site-redirect=de