The `importas` program

importas replaces a literal with the value of an environment variable, then executes another program.

Interface

     importas [ -i | -D default ] [ -u ] [ -s ] [ -C | -c ] [ -n ] [ -d delim ] variable envvar prog...

importas fetches the value of envvar in the environment. If neither the -D nor the -i option is given, and envvar is undefined, no word is returned (that is different from the empty word).
importas then performs variable substitution on prog..., with variable as key and that string as value.
importas then execs into the modified prog....

-D default : If this option is given and envvar is undefined, substitute default for the value of variable instead of no word. For instance, to substitute the empty word, use -D "".
-i : Insist. If envvar is undefined, importas will not do anything; instead, it will exit 100 with an error message. This has precedence over any -D option.
-u : Unexport. envvar will be removed from the environment after the substitution. importas -u variable envvar is equivalent to importas variable envvar unexport envvar.
Other options are used to control the substitution mechanism.

When envvar is undefined, and the -D option is not given, any variable substitution with variable as the key will return no word; that is true even when the ${variable} form to be substituted happens in the middle of a word (with a prefix and/or a postfix), which means the whole world will be deleted. If this is not the behaviour you want, use the -D option.