This is a great case for the \K assertion (update: forgot to mention that \K is new for 5.10 but available to "everyone" via Regexp::Keep by Jeff Pinyan who come up with the idea (I don't know if that will provide you the same efficiency though)). Not only is it easier, but it's also more efficient due to the optimizations of the regexp engine. The pattern would look like this:

s/\.\K[^.]*$/txt/;
[download]

The great part with this is that the engine can start looking for a literal (the dot) and avoid a lot of backtracking. The output of use re 'debug'; will visualize this.

With the look-behind pattern, you see there's a lot of backtracking going on, and the engine guesses a match at the beginning of the string (the string is "xyz.foo" in the examples below).

Compiling REx "(?<=[.])[^.]*$"
Final program:
   1: IFMATCH[-1] (7)
   3:   EXACT <.> (5)
   5:   SUCCEED (0)
   6: TAIL (7)
   7: STAR (19)
   8:   ANYOF[\0-\-/-\377{unicode_all}] (0)
  19: EOL (20)
  20: END (0)
floating ""$ at 0..2147483647 (checking floating) minlen 0
Guessing start of match in sv for REx "(?<=[.])[^.]*$" against "xyz.fo
+o"
Found floating substr ""$ at offset 7...
Guessed: match at offset 0
Matching REx "(?<=[.])[^.]*$" against "xyz.foo"
   0 <> <xyz.foo>            |  1:IFMATCH[-1](7)
                                  failed...
   1 <x> <yz.foo>            |  1:IFMATCH[-1](7)
   0 <> <xyz.foo>            |  3:  EXACT <.>(5)
                                    failed...
                                  failed...
   2 <xy> <z.foo>            |  1:IFMATCH[-1](7)
   1 <x> <yz.foo>            |  3:  EXACT <.>(5)
                                    failed...
                                  failed...
   3 <xyz> <.foo>            |  1:IFMATCH[-1](7)
   2 <xy> <z.foo>            |  3:  EXACT <.>(5)
                                    failed...
                                  failed...
   4 <xyz.> <foo>            |  1:IFMATCH[-1](7)
   3 <xyz> <.foo>            |  3:  EXACT <.>(5)
   4 <xyz.> <foo>            |  5:  SUCCEED(0)
                                    subpattern success...
   4 <xyz.> <foo>            |  7:STAR(19)
                                  ANYOF[\0-\-/-\377{unicode_all}] can 
+match 3 times out of 2147483647...
   7 <xyz.foo> <>            | 19:  EOL(20)
   7 <xyz.foo> <>            | 20:  END(0)
Match successful!
[download]

However, if we look at the \K pattern, get get this:

Compiling REx "\.\K[^.]*$"
Final program:
   1: EXACT <.> (3)
   3: KEEPS (4)
   4: STAR (16)
   5:   ANYOF[\0-\-/-\377{unicode_all}] (0)
  16: EOL (17)
  17: END (0)
anchored "." at 0 floating ""$ at 1..2147483647 (checking anchored) mi
+nlen 1
Guessing start of match in sv for REx "\.\K[^.]*$" against "xyz.foo"
Found anchored substr "." at offset 3...
Found floating substr ""$ at offset 7...
Starting position does not contradict /^/m...
Guessed: match at offset 3
Matching REx "\.\K[^.]*$" against ".foo"
   3 <xyz> <.foo>            |  1:EXACT <.>(3)
   4 <xyz.> <foo>            |  3:KEEPS(4)
   4 <xyz.> <foo>            |  4:  STAR(16)
                                    ANYOF[\0-\-/-\377{unicode_all}] ca
+n match 3 times out of 2147483647...
   7 <xyz.foo> <>            | 16:    EOL(17)
   7 <xyz.foo> <>            | 17:    END(0)
Match successful!
[download]

That's nice. No backtracking.

lodin