I had forgotten an important detail. That should be "when the old value is no longer needed and the variable name also describes the new value". This balancing act is to avoid proliferating variable names like $file, $file2, $realfile, and similar problems that I have seen in existing code, including the questioner's example 1B.

If the role of a variable can change, then (in my view, in Perl) the variable is defined in a scope that is too wide for the code as written. I often reuse the same name for another (similar) purpose later in a sub or script, for example, if iterating over two different sets of files, both foreach loops are likely to use foreach my $filename ..., but the variables are separate lexicals and $filename does not exist outside of those loops.

Thanks for catching that — the idea that a variable name must describe its contents is something that I tend to assume goes without saying and that the questioner here seems to also tacitly understand, but that is an important detail that a new programmer might not yet know.