If you're scheduling the script to run every Monday (and only on Monday, or whatever day you're after), then can just see if its the nth week of the month. Here's a version that doesn't use modules (notice there's no real difference between bad input and a day that's not the nth week):

my $nth = shift;
die "N must be numeric" if $nth =~ /\D/;
die "Day is out of range" if $nth > 5;

my ($mday)  = (localtime)[3];
my $weeks = int(($mday-1)/7)+1;
exit 0 if $weeks == $nth;
exit 1;
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