>perl -Mre=debug -e"/$_/ for qw( a a b a )" 2>&1 | find "Compiling"
Compiling REx "a"
Compiling REx "b"
Compiling REx "a"

##</code><code>##

>perl -Mre=debug -e"/a/; /a/;" 2>&1 | find "Compiling"
Compiling REx "a"
Compiling REx "a"

##</code><code>##

my $compiled_pat = $compiled_pats{$pat} ||= qr/$pat/;