This likely means that your $x and $y are not Math::BigFloat objects. Maybe you want to convert them to that?

$x = Math::BigFloat->new( $x );
$y = Math::BigFloat->new( $y );
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But most likely, you have already lost some precision you likely wanted to keep. You should convert your numbers to BigFloats at the earliest possibility.

Then you'll need to make $y a Math::BigInt object before you call methods on it:

            $y = $cd_lookup{$codon}*$mfactor;

            $y = Math::BigInt->new( $y );

            $z = $y->bnok($x);
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