Yes. Using undef as the third argument does have that effect. I suspect Perl 6 will improve on this somewhat by being able to do: substr $str, -1, replace => ''; This may not seem like much of an improvement, but it gets rid of the undef, which could possibly be mysterious to the uninitiated (as could lvalue substr).

Update: yikes! Using undef doesn't actually work like this (i.e. as the third arg in a Perl-5 substr). I had assumed it did, but it actually acts the same as using 0 as the third arg (at least with 5.8.0). Anyways, the lvalue behavior is stable and documented since 5.6 (possibly earlier); if undef works like this in any Perl version, I haven't seen it documented. Sorry for the foolish mistake (and for the unfounded assumption).