For completeness, the following code stresses out that the last compilation per code position is reused if possible.

perl -Mre=debug -e '@x=qw/a b a a c a/; while (($a,$b,@x)=@x) { /$a/;/
+$b/ }' 2>&1|grep Compiling
Compiling REx "a"
Compiling REx "b"
Compiling REx "a"
Compiling REx "c"
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Each op remembers the last pattern it compiled.

Yeah, that's kind of what I expected (was too lazy for details), but a general cache would be indeed real overkill.

Both ways there is not much use left for /o, IMHO the perlre and perlretut should be updated with at least footnote...