Following will solve December 23rd 2012 to December 23 2012. It will work for all 1st, 2nd and 3rd to 1, 2, 3 respectively.

use warnings;
use strict;

while (<DATA>) {
    s/(\d+)([a-z]{2})/$1/;
    print;
}

__DATA__
December 14th 2012
December 21st 2012
December 22nd 2012
December 23rd 2012
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