It should be quite straighforward:

sub foo
{
  my $basename = $_[0] . $_[1];
  my $addon = '0';
  while (defined($(my $goodname = "$basename$addon"))
  {
    $addon++;
  }
  my $$goodname = $_[2];
  return $goodname;
}
[download]

untested, but

$name = foo('example','other', 49);
print $$name . "\n";
[download]

should print 49 and put 49 in variable $exampleother0

Update: I use symbolic variables quite a bit for reusable code, but I agree with Fletch that you should learn to use data structures first.

2 made the code function correctly... :D