The text matched by the highest used capture group of the last successful search pattern. It is logically equivalent to the highest numbered capture variable ($1, $2, ...) which has a defined value.
This is useful if you don't know which one of a set of alternative patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.