So where is the bashism?
It's in the implementation. What odd thing do you see below?
$ strace -fqe open bash -c 'cat <<EOF
foo
EOF' 2>&1 | grep -v /lib
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/dev/tty", O_RDWR|O_NONBLOCK|O_LARGEFILE) = 3
open("/proc/meminfo", O_RDONLY|O_CLOEXEC) = 3
[pid 6696] open("/tmp/sh-thd-1384296303", O_WRONLY|O_CREAT|O_EXCL|O_TRUNC|O_LARGEFILE, 0600) = 3
[pid 6696] open("/tmp/sh-thd-1384296303", O_RDONLY|O_LARGEFILE) = 4
[pid 6696] open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
foo
--- SIGCHLD (Child exited) @ 0 (0) ---
Yes, it uses temporary files!
So do ksh, pdksh, mksh, posh and possible other shells. Busybox's sh and dash do not use temporary files, though:
$ strace -fqe open dash -c 'cat <<EOF
foo
EOF' 2>&1 | grep -v /lib
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
[pid 6767] open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
foo
--- SIGCHLD (Child exited) @ 0 (0) ---
Next time you want data to never hit a hard disk, beware that heredocuments and herestrings are best avoided.
