sub map_pairs(&@) {
  my $f = shift;
  my @res;

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

  push @res, $f->($a,$b) while ($a, $b) = splice @_, 0, 2;
  return @res;
}

##</code><code>##

map_pairs { $a } qw/a b c d/; # ac
map_pairs { $b } qw/a b c d/; # bd
map_pairs { uc($a),lc($b) } qw/a b c d/; # AbCd