qr// interpolation is regular string interpolation. They use the same operation internally and you have to alter the compiled program's optree to change this.

Make your rx/string decision at compile-time and you have to alter your program's optree so every regular string concatenation is replaced with a function which will handle your objects in a special way and everything else in a normal way. That is, every instance of concat that is not also matched by regcomp/regcreset/concat should be replaced with concat/entersub/etc.

Please bear in mind that if you alter your +imsx modifiiers that you must discard the compiled regex and compile it again with the new flags.