Interesting to notice how the collection of test cases is growing as monks review other monk suggestions. Reminds me of XP's Test-Driven Development except in reverse.

To consolidate -

__DATA__
1   32
2   20
2   21
4   19
28 31
28  39
34  123
51  1
199 200
199 201
321 321
324 329
325 349
340 509
999 1000
1000 1001
1999 2000
[download]

PS: The unusual case of  51   1  doesn't work with Abigail's code but could be fixed easily, perhaps with  ($f,$s) = ($s,$f) if $s < $f;