You might want to clarify the encoding of your example.

A fair point: it is utf-8. But I don't think that matters overly because ...

You do not use utf8;, yet you have a literal 'µ'. So, if you have an editor which saves this as UTF-8, then $micro is a string of two characters, and your substitution replaces the very same two characters with an u. This is what I'd call "works by accident".

If it works by accident then it does so deliberately :-) What I have tried to demonstrate here is that you can substitute a given symbol in code simply with a regex. But of course this depends entirely on where your string/corpus comes from, how it's encoded (or not) and how the pattern is constructed. Our anonymous OP has only provided the last of these, so it becomes guesswork in regards to what they are really doing.

And you are right about the difference between the mu and micro characters in unicode. I rarely type the micro symbol - much more frequently I write it by hand where I use a mu and always have done. The difference (to me, anyway) is a semantic one rather than a typographical one. I probably should have used µ in the example instead of μ - mea culpa.