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

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