I managed to rewrite this in a far smaller form. Also, at Damian Conway's urging, I renamed the module to Perl6::Interpolators. The pod is basically the same, with the first paragraph of 'BUGS' removed. Anyway, here's the new code, which uses Text::Balanced:

package Perl6::Interpolators;

use Filter::Simple;
use Text::Balanced qw(extract_codeblock);

FILTER {
    my($inside_stuff, $t, $pos);
    
    while(($pos=index($_, '$(')) != -1) {
        $t=substr($_, $pos);
        $inside_stuff=extract_codeblock($t, '()', qr/\$/);

        s<\$\Q$inside_stuff\E><\${\\scalar$inside_stuff}>;
    }
    
    ($inside_stuff, $t, $pos)=(undef, undef, undef);

    while(($pos=index($_, '@(')) != -1) {
        $t=substr($_, $pos);
        $inside_stuff=extract_codeblock($t, '()', qr/\@/);

        s<\@\Q$inside_stuff\E><\@{[$inside_stuff]}>;
    }
};
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