use warnings;
use strict;
use Date::Calc qw/Add_Delta_YM/;

sub prevmonth {
	my ($year, $month) = @_;
	my ($newyear, $newmonth, undef)
		= Add_Delta_YM( $year, $month, 1, 0, -1 );
	return [$newyear, $newmonth];
}

use Test::More;

is_deeply prevmonth(2003,8), [2003,7];
is_deeply prevmonth(2004,1), [2003,12];

done_testing;

##</code><code>##

use warnings;
use strict;
use Time::Piece;

while (<>) {
	chomp;
	my $t = Time::Piece->strptime($_, '%m/%Y');
	my $prev = $t->add_months(-1);
	print $prev->strftime('%m/%Y'), "\n";
}