For unix timestamps, just prepend "epoch ". This is well documented.
$ perl -wE 'use Date::Manip; say UnixDate("epoch 1264108407", "%O\n")'
+
2010-01-21T22:13:27
If you have Date::Manip 6, you can also parse that format with Date::Manip directly, though not with the default parsing rules, you have to give the format explicitly.
$ perl -wE 'use Date::Manip 6.0; my $d = Date::Manip::Date->new; $d->p
+arse_format("%a %b %d %T %Y %z", "Thu Jan 21 17:13:27 2010 -0400"); s
+ay $d->printf("%O");'
2010-01-21T17:13:27
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