...
sub convert
{
    my $re = shift;
    $re =~ s( \\ ( \\ | C\{ (?>\s*) ((?>\w+)) (?>\s*) \} ) )
            {
                defined $2
                ? "(?{\$~{$2}=\$^N})"
                : "\\"
            }xeg;
    "(?{undef(%~)})" # clear the %~
    .$re
    ."(?{\$~{\$_}=\${\$_} for(1..\$#+)})"; # add the numbered matches
}
...

my $re = qr/(\w+)\C{baz}(?: - (\w+)\C{qux})?(\+\d+)/;

"three - four - five+89" =~ $re;
print "baz=$~{baz}, qux=$~{qux}, $~{3}\n";

##</code><code>##

my $re = qr/(?\$bar=\w+) - (?\$qux{not}=\w+)/;

##</code><code>##

...
sub convert
{
    my $re = shift;
    $re =~ s<\(\?\\\$([^=]+)=([^)]*)\)><($2)(?{\$$1=\$^N})>g;
	$re
}
...

##</code><code>##

my $re = qr/...(?\$var=a(\d+|\w-\w+)b).../;