How about this:

# Prepend $pwd as necessary
@p=();
foreach (split("/", $path)) {
  pop @p, next if $_ eq '..';
  next if $_ eq '.';
  push @p, $_;
}
$path=join("/", @p);
[download]

It is clearer, in my opinion, but considerably slower:

Benchmark: timing 1000 iterations of Mikfire, Zamboni...
   Mikfire:  1 wallclock secs ( 0.47 usr +  0.00 sys =  0.47 CPU)
   Zamboni:  4 wallclock secs ( 3.23 usr +  0.00 sys =  3.23 CPU)
[download]

As given, your last solution produces double diagonals sometimes. For the string '/home/zamboni/tmp/../lib/./tex/..' it produces '/home/zamboni//lib//'. The solution is to match a second optional diagonal after the (\.)?, like this:

while ( $path =~ s#(/[^/]+)?/\.(\.)?/?#defined($2) ? "/" : "$1/"#e ) {
+;}
[download]

--ZZamboni