At a first glance, this DateTime::Format::ISO8601 should be helpful. Probably this as well: ISO 8601...

Update: Here is some sample code

use strict;
use warnings;
use DateTime::Format::ISO8601;

my $str = '2013-10-28T18:59:52.863Z';
my $dt  = DateTime::Format::ISO8601->parse_datetime( $str );
my $now = DateTime->now;

print +($dt->epoch() - $now->epoch())/3600, " hours\n";
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