The truth is that {...} in my sample code stands in for a multitude of sins. In reality, I loop through @_ looking for one I like, splice it out into a lexical, test it for writability, then stash it in caller's namespace and source filter his code to call a routine in My::Module which grabs $fh from stash, prints to it, and so forth. Deliberately, I'm not showing any of this, including the import sub, which isn't one anyway. Sorry. our $fh was just a shorthand for the wacky stash-and-grab.

I'll rewrite the example along your improved lines, since none of this has any bearing on the question of what kind of syntax caller is allowed.