=item my @result = map2 BLOCK ARRAY, ARRAY;

Similar to c<map>, but operate on two lists at once.  Inside the
BLOCK, the value are passed in as C<$a> and C<$b> (as in C<sort>).

  my @a = ( 1, 2, 5, 7, );
  my @b = ( 3, 4, 6, 8 );

  my @sums  = map2 { $a+$b } @a, @b;
  my @prods = map2 { $a*$b } @a, @b;

One use is to initialize hashes from two arrays.  You can do this in
straight perl with:

  my %hash; @hash{@hash_keys} = @values;

With this subroutine, you could also do:

  my %hash = map2 { ( $a => $b ) } @hash_keys, @values;

If the arrays are not the same length, the shorter array is used to
limit how far we go in each array.

BLOCK is evaluated in list context.

=cut

sub map2 ( & \@ \@ )
{
    my ( $code_ref, $a_ref, $b_ref ) = @_;
    my $rv_len = ( @$a_ref < @$b_ref ) ? @$a_ref : @$b_ref;
    my @rv;
    for ( my $i = 0; $i < $rv_len; ++$i )
    {
        local ( $a, $b ) = ( $a_ref->[$i], $b_ref->[$i] );
        push @rv, $code_ref->();
    }
    return @rv;
}