$ perl -Mre=debug -e'$x = "a"; for my $y (qw( b b a a )) { /$x/; /$y/; }' 2>&1 | grep -i comp
Compiling REx "a"
Compiling REx "b"
Compiling REx "a"
Skipping recompilation of unchanged REx "a"       /$x/ same as last
Compiling REx "b"
Skipping recompilation of unchanged REx "b"       /$y/ same as last
Compiling REx "a"
Skipping recompilation of unchanged REx "a"       /$x/ same as last
Compiling REx "a"
Compiling REx "a"
Skipping recompilation of unchanged REx "a"       /$x/ same as last
Compiling REx "a"
Skipping recompilation of unchanged REx "a"       /$y/ same as last