Is the problem you are having in placing this function into a module? If that is the case, then the problem is getting ahold of the caller's $a and $b. Check out the code from reduce in List::Util.

sub reduce (&@) {
  my $code = shift;
  no strict 'refs';

  return shift unless @_ > 1;

  use vars qw($a $b);

  my $caller = caller;
  local(*{$caller."::a"}) = \my $a;
  local(*{$caller."::b"}) = \my $b;

  $a = shift;
  foreach (@_) {
    $b = $_;
    $a = &{$code}();
  }

  $a;
}
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Update: D'oh, a day late and a dollar short.