G'day einhverfr,

"So how can I detect the namespace of the subclass, at runtime, which calls this function?"

It sounds like caller will do what you want. There's three ways to use this function, each returns the caller's package name as the first argument:

$package = caller;
($package, $filename, $line) = caller;
($package, $filename, $line, $subroutine, $hasargs, $wantarray,
    $evaltext, $is_require, $hints, $bitmask, $hinthash) = caller($i);
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See the doco for full details.