my has a dual nature:

• a declaration at compile-time and
• an initialisation at run-time to the assigned value (or undef if missing)

so in your code

• at compile-time every variable $test in the scope of the function (post-fix if has no own scope) is bound to a storage in the "functions-pad"
• at run-time the variable is not reset

Hence you just disabled the initialisation and the old value survives.

Rule of thumb: using my within a conditional branches without own scope is bad style and vulnerable to creating unwanted side effects!