use warnings;
use strict;

while (<DATA>) {
    s/(\d+)(th|st|nd|rd)/$1/;
    print;
}

__DATA__
December 14th 2012
December 21st 2012
December 22nd 2012
[download]

See also:

• perlre
• s///

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
[download]

Here is one way:

#! perl
use Modern::Perl;

my $a = "December 14th 2012";
my $b = "December 21st 2012";
my $c = "December 22nd 2012";

for ($a, $b, $c)
{
    my @fields = split;
    $fields[1] =~ s/st|nd|rd|th//;
    $_ = join(' ', @fields);
    say;
}
[download]

Output:

12:58 >perl 433_SoPW.pl
December 14 2012
December 21 2012
December 22 2012

12:58 >
[download]

Hope that helps,

use Modern::Perl;
use Date::Parse;
use DateTime;

while (<DATA>) {
    my $dt = DateTime->from_epoch( epoch => str2time($_, '+0000'));
    say join ' ', $dt->month_name(), $dt->day(), $dt->year();
}

__DATA__
December 14th 2012
December 21st 2012
December 22nd 2012
[download]

December 14 2012
December 21 2012
December 22 2012
[download]

Treating the data as a date rather than a generic string can have a huge advantage in avoiding unwanted changes.
Bill

  DB<101> s/(\d)\w\w /$1 / for @a=("December 14th 2012", "December 21s
+t 2012" , "December 22nd 2012")
 => ""
 
  DB<102> @a
 => ("December 14 2012", "December 21 2012", "December 22 2012")
[download]

UPDATE:

a bit more fail safe!

  DB<126> map {/^(\w+ \d\d?)\w\w( \d\d\d\d)$/; "$1$2"} "December 1th 2
+012", "December 21st 2012" , "December 22nd 2012"
 => ("December 1 2012", "December 21 2012", "December 22 2012")
[download]

What the heck. Another approach:

>perl -wMstrict -le
"my @dates = (
   'December 1st 2012',    'December 2nd 2012',
   'December 23rd 2012',   'December 24th 2012',
   'December 765th 2012',  'an 8th note',
   );
 ;;
 for my $date (@dates) {
   printf qq{'$date' -> };
   $date =~ s{ \d \K  (?: st | nd | rd | th) \b }{}xms;
   print qq{'$date'}
   }
"
'December 1st 2012' -> 'December 1 2012'
'December 2nd 2012' -> 'December 2 2012'
'December 23rd 2012' -> 'December 23 2012'
'December 24th 2012' -> 'December 24 2012'
'December 765th 2012' -> 'December 765 2012'
'an 8th note' -> 'an 8 note'
[download]

Dear Monks, I am not a seasoned Perl Programmer, so am asking your wisdom. Be Kind Monks.

FYI, this is first day of class kind of stuff, read perlintro#Regular expressions and perlrequick#Search and replace