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

   our @args;
   (local *_, local *args) = (sub { \@_ }->($a, $b), \@_);

   no strict 'refs';
   my $caller = caller();
   local *{$caller."::a"} = \$a;
   local *{$caller."::b"} = \$b;

   $b = shift(@args), $a = &$f while @args;
   return $a;
}