(caller)[3] eq 'fib' or ($nth !~ m{^\d+\z}a or @_) and die $fib_die_msg; #### use strict; use warnings; use 5.20.0; use Attribute::Handlers; package Uint { require Tie::Scalar; @Uint::ISA = qw(Tie::StdScalar); sub STORE { my ($p,$f,$l) = caller; $_[1] !~ /^\d+\z/a and die "not a positive integer in package $p file $f line $l, aborted"; ${$_[0]} = $_[1]; }; } sub UNIVERSAL::Uint : ATTR(SCALAR) { tie ${$_[2]}, 'Uint' } sub fib { my $nth : Uint = shift; # check for extra items in @_ missing return $nth if $nth <= 1; return fib( $nth - 1 ) + fib( $nth - 2 ); } say fib @ARGV; #### sub fib :prototype($) ($nth : Uint) { # check for 1 == @_ already done ... }