Let the effort be in explaining this code, if the AM ever uses it.

#!/usr/bin/perl
# There is everything you need:
# a sentinel,
$f{0} = sub { 1 };
# a recursive function,
sub f {
  $f{$n = pop()-1} ||= sub { f($n) };
  ($n+1) * $f{$n}->();
}
# a call to the recursive function...
print f(shift);
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I hope it doesn't need to pass -w and use strict... ;-) And what did he mean, test if the parameter is a number?!