I recommend to use ${^MATCH}.

Always use the /p regex modifier when using  ${^MATCH} and friends. The use of  ${^MATCH} may appear to work without the /p modifier, but this is only fortuitous.

Quoth perlvar (emphasis added):

${^MATCH}

This is similar to $& ($MATCH) except that it does not incur the performance penalty associated with that variable, and is only guaranteed to return a defined value when the pattern was compiled or executed with the "/p" modifier.