Yes, a hash slice (or a loop) is the way to go, imho.

Assuming you have the new values you want in %newbits:

@bighash{keys %newbits} = values %newbits;
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or

$bighash{$_} = $newbits{$_} for keys %newbits;
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Note that keys and values use the same order, so the above works.