Programming Perl, 4th Edition

Dynamically Scoped Variables: local

Using a local operator on a global variable gives it a temporary value each time local is executed, but it does not affect that variable’s global visibility. When the program reaches the end of that dynamic scope, this temporary value is discarded and the original value is restored. But it’s always still a global variable that just happens to hold a temporary value while that block is executing. If you call some other function while your global contains the temporary value and that function accesses that global variable, it sees the temporary value, not the original one. In other words, that other function is in your dynamic scope, even though it’s presumably not in your lexical scope.^[83]

This process is called dynamic scoping because the current value of the global variable depends on your dynamic context; that is, it depends on which of your parents in the call chain might have called local. Whoever did so last before calling you controls which value you will see.

If you have a local that looks like this:

{
  local $var = $newvalue;
  some_func();
  ...
}

you can think of it purely in terms of runtime assignments:

{
  $oldvalue = $var;
  $var = $newvalue;
  some_func();
  ...
}
continue {
  $var = $oldvalue;
}

The difference is that with local the value is restored no matter how you exit the block, even if you prematurely return from that scope.

As with my, you can initialize a local with a copy of the same global variable. Any changes to that variable during the ...