# some good reasons:

# if you modify a side-effect global like $/, 
# do it in the smallest scope possible:
my $slurped = do { local $/; <$some_Fhand> };

# override an element of a global aggregate in a 
# limited scope:
{
   local $SIG{"ALRM"} = \&some_handler;
   alarm $timeout;
   some_action();
}