Note that there is no need to track stack depth independently in Perl if you use an array as your stack with the push and pop builtins: Perl will keep track of the array length, which is accessible either by using the array in scalar context or with the $# sigil. There is an important difference: scalar @array gives the number of elements in @array (0 if @array is empty, 1 if there is one element) while $#array gives the highest index in @array (-1 if @array is empty, 0 if there is one element).