[QandAEditor's note: the answer that had used ONE_MONTH has been fixed. Thanks to swngnmonk for pointing this out]

A note on Time::Piece - While I love it, and I use it heavily, the ONE_MONTH constant is fundamentally broken, because there's no such thing as a constant value for a month.

For example:

#!/usr/bin/perl

use Time::Piece;
use Time::Seconds; 

my $now = localtime;
 
print sprintf("Now = %s\n", $now->cdate);
 
for (1..12) {
  $now += ONE_MONTH;
  print sprintf("Now + %02i months = %s\n", $_, $now->cdate);
}
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Outputs something like this:

bash-2.05b$ ./broken.pl
Now = Mon May 10 15:56:13 2004
Now + 01 months = Thu Jun 10 02:25:17 2004
Now + 02 months = Sat Jul 10 12:54:21 2004
Now + 03 months = Mon Aug  9 23:23:25 2004
Now + 04 months = Thu Sep  9 09:52:29 2004
Now + 05 months = Sat Oct  9 20:21:33 2004
Now + 06 months = Tue Nov  9 05:50:37 2004
Now + 07 months = Thu Dec  9 16:19:41 2004
Now + 08 months = Sun Jan  9 02:48:45 2005
Now + 09 months = Tue Feb  8 13:17:49 2005
Now + 10 months = Thu Mar 10 23:46:53 2005
Now + 11 months = Sun Apr 10 11:15:57 2005
Now + 12 months = Tue May 10 21:45:01 2005
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Originally posted as a Categorized Answer.