>perl -we "if (undef = $foo) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.
Modification of a read-only value attempted at -e line 1.

>perl -we "if ($foo = undef) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.