Your approach was exactly what the OP was looking for (++), but your example has some excess baggage that made it confusing for me. Simplified:

sub myjoin (&@) {
  my $sub = shift;
  my $res = shift;
  $res .= $sub->() . $_ for (@_);
  $res;
}
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Update: Oh, I see: the arguments to sub were to stand in for $a and $b (a la sort), in case the user wanted to have that kind of flexibility in the sub.

Is the thread title a C.S. Lewis reference, I wonder?