>perl -wMstrict -le
"sub fact ($);  # function declaration
 print qq{$_! == }, fact($_) for 0 .. 5;
 sub fact ($) {  # function definition
   my $n = shift;
   return $n > 1 ? $n * fact($n - 1) : 1;
   }
"
0! == 1
1! == 1
2! == 2
3! == 6
4! == 24
5! == 120