In Perl6, the special MAIN subroutine, if any, receives the arguments passed to the program and can be used to check their type and validity:

~ perl6 -e 'sub MAIN (Int $month where 1 <= $month <= 12) {say "Succes
+s";}' 4
Success

~ perl6 -e 'sub MAIN (Int $month where 1 <= $month <= 12) {say "Succes
+s";}' 14
Usage:
  -e '...' <month>

~ perl6 -e 'sub MAIN (Int $month where 1 <= $month <= 12) {say "Succes
+s";}' 8.5
Usage:
  -e '...' <month>
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