The Date::Manip module accepts that format of dates by default (that is, it parses it fine without having to tell it the format), so let's use that.

$ perl -we 'use Date::Manip; print UnixDate($ARGV[0], "%Q\n")' 11004
20110104
$ perl -we 'use Date::Manip::Date 6.0; print Date::Manip::Date->new($A
+RGV[0])->printf("%Q\n")' 11004
20110104
$
[download]