Wow, learn something new every day. I looked it up in perldoc perlop and found this relevant bit:

Non-bracketing delimiters use the same character fore and
aft, but the four sorts of brackets (round, angle, square,
curly) will all nest, which means that

  q{foo{bar}baz}

is the same as

  'foo{bar}baz'
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And if I'd taken more than a second to glance at it, I would have noticed the non-metacharacter '.' inside the square brackets. I'm familiar with that particular pitfall, having done it in my own code many times. But thanks for pointing it out.