A quick read of the POD suggests you should use add_months() and add_years() to get accurate values. Adding the following just before your __DATA__ works for me on Windows 7 x64 / Strawberry 5.18.1.

print "Done according to POD:\n";
print "  Mon: " . $now->add_months(-1) . "\n";
print " Year: " . $now->add_years(-1) . "\n";
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UPDATE:

I don't need Time::Piece to verify that I'm 10 minutes too late as Toolic (above) beat me to it.