A simple my %copy = %+; is equivalent to your sub. However, that may not be enough!
According to the docs for Tie::Hash::NamedCapture, the values of the tied hash can contain an array reference holding multiple captures due to the re-use of the same named capture. In which case you might need to use something more complex to perform a deep copy operation.
That said, in my simple tests, this doesn't seem to apply to %+. It does apply to %- though.