The following code works perfectly here.

#!/usr/bin/perl

use strict;
use warnings;
use CGI qw(:standard);
use CGI::Carp qw(fatalsToBrowser warningsToBrowser);

my $shop = lc(param('shop'));

if ($shop eq "rg") {
    print redirect('https://www.planetdistribution.co.uk/store/index.p
+hp?cPath=6');
} elsif ($shop eq "ps") {
    print redirect('https://www.planetdistribution.co.uk/store/index.p
+hp?cPath=1');
} elsif ($shop eq "sk") {
    print redirect('https://www.planetdistribution.co.uk/store/index.p
+hp?cPath=19');
} elsif ($shop eq "smm") {
    print redirect('https://www.planetdistribution.co.uk/store/index.p
+hp?cPath=9');
} else {
    print redirect('https://www.planetdistribution.co.uk/store');
}
[download]

HTTP Error 500 is an Internal Server Error if I'm not mistaken, so the problem lies in your web server configuration. As always, check the error_log.