Rvalue references
From the C++11 FAQ, section on rvalue references, about a simple incrementation function incr:
If that incr(0) were allowed either some temporary that nobody ever saw would be incremented or - far worse - the value of 0 would become 1. The latter sounds silly, but there was actually a bug like that in early Fortran compilers that set aside a memory location to hold the value 0.
That actually made me think about that nifty one-liner:
perl -wE 'Internals::SvREADONLY(${\undef},0);undef=42;say undef'
I can haz Perl.
Rvalue references are more awesome than most people realize.
As for your one-liner, I honestly think that that should be disallowed, at least for "immortals".
$ perl -wE 'Internals::SvREADONLY(${\undef},0);undef=42;say undef; undef $^O; say $^O'
42
Use of uninitialized value $^O in say at -e line 1.
@Burak:
The code sets the undef variable (that secretly exists behind the undef keyword), it will not change any other undefined value.
Right... I thought it was also creating a glob at the same time (which I'd expect to override undef()).