Inside the (?{...}) block, $_ refers to the string the regular expression is matching against. You can also use pos() to know what is the current position of matching withing this string. 

+ Currently, lexical (my) variables within a (?{...}) block can be
+ problematic, as their use may lead to the unintentional creation of
+ a lexical closure.  For that reason it is usually advisable to
+ use globals in (?{...}) blocks.

The code is properly scoped in the following sense: If the assertion is backtracked (compare "Backtracking"), all changes introduced after localization are undone, so that