My mistake was thinking that RE/o was compiled once in its narrowest scope - that the subroutine could process 1000 files with one fixed RE, and then process another 1000 files with a different RE.

Perl does that without the 'o' modifier.

for my $re (qw( a a a b b a )) {
   '' =~ /$re/;
}
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>perl -Dr 731397.pl 2>&1 | find "Compiling REx"
Compiling REx `a'
Compiling REx `b'
Compiling REx `a'
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Notice how the regexp is only compiled whenever the interpolated variable changes. (Tested 5.6.*, 5.8.*, 5.10.0)