How very odd!

My testing on Win2k, ActivePerl 5.8.8 shows that the case of the data doesn't affect the timing, and that old_way is consistently just shy of 50% faster than with_tr.

Upper case data
===============
            Rate with_tr old_way
with_tr 183850/s      --    -32%
old_way 272269/s     48%      --

Lower case data
===============
            Rate with_tr old_way
with_tr 182262/s      --    -33%
old_way 270854/s     49%      --
[download]

Benchmark code:

Of course, inlining the code saves a lot of time:

              Rate   with_tr   old_way with_tr_i old_way_i
with_tr   180473/s        --      -32%      -39%      -52%
old_way   265506/s       47%        --      -10%      -29%
with_tr_i 296058/s       64%       12%        --      -20%
old_way_i 372377/s      106%       40%       26%        --
[download]

Benchmark code: