How do i get "the last day of last month"?

I think that the easiest way to do this involves Date::Tie.

use Date::Tie;

tie my %date, 'Date::Tie';

$date{monthday} = 1;
$date{monthday}--;

print "$date{year}-$date{month}-$date{monthday}\n";
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Juerd # { site => 'juerd.nl', plp_site => 'plp.juerd.nl', do_not_use => 'spamtrap' }