You may like some of the date-related modules mentioned on The Perl Advent Calendar. See the articles on Date::Parse and DateTime.

However, my absolute favorite modules for this is Date::Manip. It does an amazing job of recognizing almost any date format. Here's some working code for your example:

$ perl -MDate::Manip -e'print UnixDate("20050519112656","%m-%d-%Y %I:%
+M %p %Z\n")'
05-19-2005 11:26 AM EDT
[download]

Notice that I didn't have to say what format the input was in!