I am on a 64 bit machine

If there is any chance that your strings will get bigger than 4GB, I think you just need to change the if for a while and the rest would take care of itself:

sub myvec(\$$$) :lvalue  {
    use constant TWO_GB => 2**31;
    my( $ref, $offset, $bits ) = @_;
    while( $offset > TWO_GB - 1 ) {
        $offset -= TWO_GB;
        $ref = \substr $$ref, ( TWO_GB * $bits ) / 8;
    }
    CORE::vec( $$ref, $offset, $bits );
}
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but again that's untested, so check it and convince yourself.