From perldoc perlsub, section: Temporary Values via local()

A `local' modifies its listed variables to be "local" to the enclosing block, `eval', or `do FILE' --
and to any subroutine called from within that block.

But knowing this is not enough, you have to think about the implications as well!

Recently I had a hard time to find a bug in a Tk application.

Part of the application scans large textfiles. If a certain string is found, it is inserted into a listbox.
During that scan, you should be able to use the application in a normal way.
This is easyly achieved, you just have to do a $MW->update() frequently and all Tk events will be processed.

So far no problem.
But when you change the input line separator $/ to something really weird for those scan, you've got a PROBLEM!
Even if you do it local!

Because local means local to the block and all called subroutines, of which $MW->update() is one, and all subs called during the event processing also.
So every reading from files done somewhere in the application went gaagaa during those scans.

I spent hours!