Hard to know where to start here. Firstly, you seem to have completely misunderstood the DateTime::Format::Builder tutorial. Please read it again. More carefully this time.

Secondly, even when I rewrote your example to use the module correctly, it still didn't work as your regex didn't come close to matching your date string. Having corrected that as well, the code I ended up with was:

#!/usr/bin/perl

use strict;
use warnings;

package DateTime::Format::MyParser;

use DateTime::Format::Builder (
  parsers => {
    parse_datetime => {
      regex  => qr/^(\d?\d)\s(\d\d)\s(\d\d\d\d) (\d\d):(\d\d):(\d\d)$/
+,
      params => [ qw( day month year hour minute second ) ],
    },
  },
);

package main;

my $date = "Thu, 7 Aug 2008 08:35:36 +01:00";

$date =~ s/Thu, //;
$date =~ s/\s+\+01:00$//;
$date =~ s/Aug/08/;

my $parser = DateTime::Format::MyParser->new();

my $dt = $parser->parse_datetime($date);

print $dt;
[download]

It still bothers me that you do all that munging of the input string before trying to use it. Surely the point of parsing a date string is to parse the string that you are given, not to have to munge your string into some intermediate format first.

Finally, I think that DateTime::Format::Builder might be completely the wrong tool for what you are doing here. DateTime::Format::Builder is for building DateTime::Format modules (there's a clue in the name). If you're just want to parse a date string and turn it into a DateTime object, then you probably want DateTime::Format::Strptime instead.