Cool!

It seems to depend on whether chr(0), chr(1), and chr(2) are in your string. I'm guessing those are used as "magic" during the tr?

In any case, try these tests:

$_ = join '', map chr(), 1 .. 0x1f;
tr [\3\5\7]
   [\10\10\10]cd;
print join('', map sprintf("\\%o",ord), split //), $/;
# prints \10\10\3\5\7

$_ = join '', map chr(), 2 .. 0x1f;
tr [\3\5\7]
   [\10\10\10]cd;
print join('', map sprintf("\\%o",ord), split //), $/;
# prints \10\3\5\7


$_ = join '', map chr(), 3 .. 0x1f;
tr [\3\5\7]
   [\10\10\10]cd;
print join('', map sprintf("\\%o",ord), split //), $/;
# prints \3\5\7

   $_ = join '', reverse map chr(), 0,0,0,1,0,1,0,1,0 .. 0xA;
   tr [\3\5\7]
      [\10\10\10]cd;
      print join('', map sprintf("\\%o",ord), split //), $/;
#prints \7\5\3\10\10\10\10\10\10\10\10\10\10\10
[download]

It also appears that if you use more characters in the left hand side of the tr///, more of the "low byte values" become magical.

Sorry I can't tell you WHY it happens, but I hope this helps...
--
Mike