I don't know the answer to your question, but the POD does mention add_months and add_years, which seem to work as you want: 
use strict;
use warnings;
use Time::Piece;
use Time::Seconds;

my $now = localtime;
print "Today: $now\n";
print " Week: ", $now - ONE_WEEK, $/;
print "  Mon: ", $now - ONE_MONTH, $/;
print "  Mon: ", $now->add_months(-1), "\n";
print " Year: ", $now - ONE_YEAR, $/;
print " Year: ", $now->add_years(-1), "\n";

__DATA__
Today: Fri Oct  2 16:12:17 2015
 Week: Fri Sep 25 16:12:17 2015
  Mon: Wed Sep  2 05:43:13 2015
  Mon: Wed Sep  2 16:12:17 2015
 Year: Thu Oct  2 10:23:27 2014
 Year: Thu Oct  2 16:12:17 2014

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In reply to Re: Arithmetic bug within Time:Piece? by toolic
in thread Arithmetic bug within Time:Piece? by jeffa

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