Because I recently learned about regexp_pattern (see thread), this might work, but it might not handle every case (there are other flags besides "u"):

use re ();
for (1..10) {
    my ($pat, $flags) = re::regexp_pattern($rex);
    $rex = ($flags eq "u" or $flags eq "") ? qr{$pat.} : qr{$rex.};
}
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