use strict;

eat('Ahh ... recursion!');

sub eat {
   my $food = shift;
   if ($food) {
      print "$food\n";
      eat (substr($food,0,length($food)-1));
   }
   else {
      # base case
      return;
   }
}

##</code><code>##

use strict;

my @stack = ('Ahh ... recursion!');

eat() while @stack;

sub eat {
   my $food = pop @stack;
   if ($food) {
      print "$food\n";
      push @stack, substr($food,0,length($food)-1);
   }
}

##</code><code>##

use strict;

my $numb = shift || 20;

print fib($numb), "\n";

sub fib {
   my $n = shift;
   return 1 if $n < 2;
   return fib($n - 2) + fib($n - 1);
}

##</code><code>##

if (length $food) ...

# instead of
if ($food) ...