let's say you convert this (working) code:

$_ = "abcdefg";
my @some_array = (1 ..2);
while (m/./g) {
   for (@some_array) {  # implicitly localized - works
        # do something;
   }
   print $&;
}
[download]

$_ = "abcdefg";

while (m/./g) {
   my @some_array = (1 ..2);
   while (local $_ = shift @some_array) { # manually localized - break
+s
        # do something;
   }
   print $&;
}
[download]

Some of the parsing code i've written makes heavy use of $_ and then you can run into this problem when you're refactoring.

update: the above code breaks, because it's not the pos() function that's broken, but the match position field itself gets reset by localizing a variable - in other words, the while (m/./g) statement will loop forever.

Not an actual usage, but I could easily see something which did a match, called a sub which localized $_, and then tried to use pos() after that sub returned. Granted it'd be kinda sloppy code (like depending on $1 and friends to remain set rather than saving values off if you need to use them), but again I could see it happening.