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use strict;

...
      return;
   }
}

use strict;

...
      push @stack, substr($food,0,length($food)-1);
   }
}

use strict;

...
   return 1 if $n < 2;
   return fib($n - 2) + fib($n - 1);
}

if (length $food) ...

# instead of
if ($food) ...