Until a better version of vec is available, you might try something like this:

sub myvec(\$$$) :lvalue  {
    use constant TWO_GB => 2**31;
    my( $ref, $offset, $bits ) = @_;
    if( $offset > TWO_GB - 1 ) {
        $offset -= TWO_GB;
        $ref = \substr $$ref, ( TWO_GB * $bits ) / 8;
    }
    CORE::vec( $$ref, $offset, $bits );
}
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Which should be reasonably efficient as it avoids copying the huge string. If you wanted to get fancy in anticipation of the fixed version, you could stick it in a module and export it as CORE::GLOBAL::vec.

The above is untested at the transition limit as I don't have enough memory to create strings that big. You might want to look closely at that TWO_GB - 1...