>perl -Mre=debug -e"/$_/ for qw( foo bar bar foo )" 2>&1 | find "Compiling"
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `foo'

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

##</code><code>##

>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)'