What matters is the value of $word, because we're using the value of $word to compile the regexp. While the value of the variable to which $word is/was aliased doesn't change, the value of $word itself does change.

In the first pass, $word is "foo". The regexp was thus compiled with to /foo/. In the second pass, $word is "bar". We obviously need to recompile the regexp because we want /bar/ and it's currently /foo/. Whether $words[0] and $words[1] changed or not is completely irrelevant.

You might be thinking that the compiled regexp is stored with the variable used in the regexp. It's not. That wouldn't work when no variables or multiple variables are used to create the regexps. Instead, the uncompiled regexp or the values of the variables used to compile the regexp -- I don't know which -- is stored along with the compiled regexp in the code.