There is no perfect solution. "Matching the indent level" of a mixture of blanks and tabs relies on knowing where the tab-stops are. We could add a bunch more options to support the notion of tab-stops, as perltidy does, but that doesn't protect you from an inadvertent (or deliberate) leading space that turns a line into a verbatim line when that wasn't the intent. For what I have been trying to do, verbatim lines are relatively uncommon, so I don't mind going a little out of my way to make other stuff foolproof, if that makes verbatim lines a bit clumsier.
Given the response to this thread that could most charitably be described as "tepid", I have what I need, and I'm pleased that I'm not reinventing some existing wheel. If you have a preference for an alternative that works better for you, go for it. I'm not eager to push the idea much further.