TIMTOWTDI. You may roll your own altjoin() sub, e.g.:

sub altjoin ($$@) {
    my $d1=shift;
    my $d2=shift;
    my $cnt;
    @out=map { $_ => ++$cnt % 2 ? $d1 : $d2} @_;
    pop @out;
    @out;
}
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Update: except that this is not a "join", still I hope that as a minimal example it is not that bad...