As I see it you have two options: one that requires 'no strict qw/refs/' and one that works fine under use strict. Both of these use a simple foreach loop. (both of these examples are similar to the one ikegami posted in the read-more tags.)

Or do you really want a solution that uses map?

It is also my opinion that using eval for this is simply a bad idea... why use a complex, slow and dangerous (if used incorrectly) function for something soo simple?

First one: uses soft-reference, and does not work with lexical variables

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

Second one:

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

(You could write this one using one long list aswell, but then you would need to use a C-style for loop.)