I think the main potential problem would be when you're using localized $_ combined with m//g matches.

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]

to this (stupid but more or less equivalent) code:

$_ = "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.