$ perl -we'use 5.10.0; say 1, $x'
Name "main::x" used only once: possible typo at -e line 1.
Use of uninitialized value $x in say at -e line 1.
1
$ perl -we'use 5.12.0; say 1, $x'
Global symbol "$x" requires explicit package name (did you forget to declare "my $x"?) at -e line 1.
Execution of -e aborted due to compilation errors.