I also think this is strange. There is no checking for ref($class). I guess something like ...

    if ( ref($class) ){
        my $t = $wanted; 
        $t =~ s/^[+-]//;

        return bless {
            sign => $class->{sign},
            value => $CALC->_new($t),
        }, ref($class);
    }
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should be in new constructor, doesn't it?