On subroutine return, any return values are copied. [emphasis mine]

D'oh! Thank you for the enlightenment on a Friday afternoon ;-)

Update: Yep:

$ perl -wMstrict
*a = \substr(undef,0);  # alias via glob
$b = $a;                # copy
$a = 'x';               # assignment
__END__
Use of uninitialized value in scalar assignment at - line 2.
Modification of a read-only value attempted at - line 3.
[download]

And just like above, on Perls <5.16 that dies with "Modification of a read-only value attempted at - line 1." (instead of line 3). So I should probably call the commit I referenced a bugfix! :-)

Additional updates: Expanded the code example from the original version and added (and edited) the last paragraph.