Using a here-doc for `sed` and a file

You can tell GNU sed to read the script from standard input with -f -, -f meaning to read the script from a file, and - meaning standard input as is common with a lot of commands.

sed -f - "$IN" > "$OUT" << SED_SCRIPT
    s/a/1/g
    s/test/full/g
SED_SCRIPT

POSIX sed also supports -f, but the use of - for standard input is not documented. In this case, you could use /dev/stdin on Linux systems (and I seem to recall Solaris has this too, but I cannot confirm that right now)

Using <<-SED_SCRIPT (with the '-' prefix) will allow the closing SED_SCRIPT tag to be indented.

In case sed does not support reading of a script from stdin (using -f -), you can use process substitution (available in bash, zsh, ksh93):

sed "$IN" > "$OUT" -f <( cat << SED_SCRIPT
    s/a/1/g
    s/test/full/g
SED_SCRIPT)

The closing parenthesis ) must follow the end delimiter (SEC_SCRIPT) immediately or after a newline. In the case of process substitution you can also use echo instead of a here document:

sed "$IN" > "$OUT" -f <( echo \
"    s/a/1/g
    s/test/full/g" )