This will do what you're after in your example.

use Date::Manip;
my $str_date = shift || die "No date!\n"; # almost *any* format
my $date = UnixDate( ParseDate( $str_date ), "%b %e, %Y %r");
$date || die "Could not interpret date: $str_date!\n";
print $date, "\n";
[download]

You could also try to write up a regex/sub to check based on what the RSSes are supposed to follow, RFC #822, but I'm sure there are RSSes that break it.

One could argue that using Date::Manip to allow arbitrary dates (eg, it will even work on things like "10 days ago at noon UT") is really a bad idea b/c it allows RSS writers to get away with breaking the specification. And Date::Manip is slow but unless it's being run off a highly trafficked CGI it probably doesn't matter.