You need to clone() $first: 
...
my $last  = $first->clone()->add( months => 1 )->subtract( days => 1 )
+;

[download]
Otherwise $first and $last will be the same object.

In reply to Re: set the first day of last month to 1 by pc88mxer
in thread set the first day of last month to 1 by Ace28

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