bash uses tmp files for inter-process communication instead of pipes?

[Linda Walsh]

In running a startup script, I am endeavoring not to use tmp files
where possible, As part of this, I sent the output of a command
to stdout where I read it using the "variable read" syntax:

   while read ifname hwaddr; do
       printf "ifname=%s, hwaddr=%s\n" "$ifname" "$hwaddr"
       act_hw2if[$hwaddr]="$ifname"
       act_if2hw[$ifname]="$hwaddr"
       printf "act_hw2if[%s]=%s, act_if2hw[%s]=%s\n" 
"${act_hw2if[$hwaddr]}" "${act_if2hw[$ifname]}"
   done <<<"$(get_net_IFnames_hwaddrs)"

Note, I used the <<<"$()" form to avoid process substitution -- as I was 
told
on this list that the <<< form didn't use process substitution.

So I now get:
> >>/etc/init.d/boot.assign_netif_names#192(get_net_IFnames_hwaddrs)> 
echo eth5 a0:36:9f:15:c9:c2
/etc/init.d/boot.assign_netif_names: line 203: cannot create temp file 
for here-document: No such file or directory

Where am I using a HERE doc?

More importantly, at this point, where is it trying to write?/(Read).

Simple Q.  Why isn't it using some small fraction of the 90+G of memory
that is free for use at this point?

This really feels like "Whack-a-mole"....