$ perl -Mre=debug -c -e'/a/'
Compiling REx "a"
Final program:
   1: EXACT <a> (3)
   3: END (0)
anchored "a" at 0..0 (checking anchored isall) minlen 1
-e syntax OK
Freeing REx: "a"

##</code><code>##

$ 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