DateTime::Format::Natural can parse the string for you. You don't have to reinvent the wheel. Try this:

#!/usr/bin/perl -l

use strict;
use warnings;
use DateTime;
use DateTime::Format::Natural;

my $parser = DateTime::Format::Natural->new;
my $extract_string = "Date: 02.07.2011 16:14:48";
my $date_string = $parser->extract_datetime($extract_string);

my $dt = $parser->parse_datetime($date_string);
my $epoch_time = $dt->epoch;
print $epoch_time;
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