Um, if you mean doing length($str) requires a call to strlen(), you're wrong. From 'perlguts':

You can get and set the current length of the string stored in an SV with the following macros:

  SvCUR(SV*)
  SvCUR_set(SV*, I32 val)
[download]

And checking with Devel::Peek:

jeffp@hut [10:44am] ~ #104> perl -MDevel::Peek
$str = "something";
Dump($str);
SV = PV(0xa3424) at 0xb4fc0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0xb9600 "something"\0
  CUR = 9
  LEN = 10
[download]

So Perl determines the string length when it's set (which would be when you created the string originally), and uses that value when you call length(). But you're right that you have to go through the entire string when you use tr///. I admit that sucks a bit.