This is similar to $' ($POSTMATCH) except that it does not incur the performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a defined value when the pattern was compiled or executed with the /p modifier. In Perl v5.20, the /p modifier does nothing, so ${^POSTMATCH} does the same thing as $POSTMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.