It is my understanding that this is precisely what recent Perl implementations do.

Then your understanding is flawed. There's a cache, but it only holds one value: the last regexp compiled for that operator instance.

>perl -Mre=debug -e"qr/$_/ for qw( foo bar bar foo )" 2>&1 | find "Com
+piling"
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `foo'
[download]