my has compile-time *and* run-time effects. At compile time, the compiler knows about the variable, etc. At run-time, values are set, my $x; makes that $x becomes undef. So far, so good.

However, for efficiency reasons, if Perl exits a block, it will actually *not* delete any variables lexical to the block. You cannot refer to them anymore (the compiler takes care of that), but the data structure build for it remains. Perl does this because it is likely that you reenter a block and if the structure remains, Perl can save time rebuilding it. However, with my $x if undef, no run-time effect on $x happens when reentering the block. (The first time the block is entered, the datastructure gets build when $x is used). And since the structure doesn't get rebuild, the value doesn't get reset either. So, you have created a static variable....

-- Abigail