>perl -Mre=debug -e"$re=qr/foo/; /$re/" 2>&1 | find "Compiling"
Compiling REx `foo'

>perl -Mre=debug -e"$re=qr/foo/; /a$re/" 2>&1 | find "Compiling"
Compiling REx `foo'
Compiling REx `a(?-xism:foo)'