I would prefer to rely on a module than string-splitting.

We don't need a module to parse the grammar I suggested. The grammar is just that simple. That's the point.

All we're doing is parsing the provided value to separate it into a name and a value. And we know the names can't contains spaces or punctuation (except perhaps _ or -).

This is the same approach curl uses.

curl ... -H 'Header1: ...' -H 'Header2: ...' ...
[download]

You could leave out the initial dash if you want (-X 'y'/-X 'y ...').

You could use equal signs instead of spaces if you want (-X 'y'/-X 'y=...').

Whatever. The specifics aren't important and are quite flexible since all you have to do is separate a name (with a very limited set of characters) from the rest.

The solution I presented does not "open a different can of worms". It in fact avoids doing so.