FYI, if you want to use DateTime the way you were using it, there's clone:

#!/usr/bin/perl

use DateTime;

$dt = DateTime->now();
$month = $dt->clone->subtract(months => 1)->strftime("%m");
$year = $dt->clone->subtract(months => 1)->year();

print $dt."\n";

print $month."\n";

print $year."\n";
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