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;
}

[download]

Update: except that this is not a "join", still I hope that as a minimal example it is not that bad...

In reply to Re: how to alternate join? by blazar
in thread how to alternate join? by SamCG

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