use strict;
use warnings;

sub reduce(&@) {
   my $f = shift;
   my $a = shift;

   if (prototype($f)||'' eq '$$') {
      $a = $f->($a, shift) while @_;
   } else {
      my $b;
      no strict 'refs';
      my $caller = caller();
      local *{$caller."::a"} = \$a;
      local *{$caller."::b"} = \$b;
      $b = shift, $a = $f->() while @_;
   }

   return $a;
}


sub prototyped($$) { $_[0] * $_[1] }
sub unprototyped   { $a * $b }


$\ = "\n";
print reduce { $a * $b }     1..6;  # 720
print reduce \&prototyped,   1..6;  # 720
print reduce \&unprototyped, 1..6;  # 720