However the compiler cannot stitch together subpatterns in compiled form, so whenever you build up a pattern from two previously qr'd patterns, or a qr'd pattern and some new text, perl constructs the stringified form of the new pattern and then compiles this new string from scratch.

Interesting! I wonder if there is a need for a low-level regex-manipulating package that would allow us to do manipulations like this on the actual regex objects. (Or if such an interface already exists!)

One comment / question on your preferred solution:

my $days_re = join '|', map "\Q$_\E", @days;
[download]

I've seen this used a few times now, and I wonder if there is a meme out there people are copying from. Is there any reason to prefer this over:

my $days_re = join '|', map quotemeta, @days;
[download]

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