no strict qw/refs/;
foreach my $e (qw/a c b/) {
  if ($$e) {
    $combined .= $e . "=" . $$e;
  }
}

##</code><code>##

foreach my $e ( ["a", $a], ["c", $c], ["b", $b]/) {
  if ($e->[1]) {
    $combined .= $e->[0] . "=" . $e->[1];
  }
}