PATTERN may contain variables, which will be interpolated every time the pattern search is evaluated, except for when the delimiter is a single quote.
Parens aren't the only thing you have to look out for. '.' (dot) will be treated as a meta-character that matches anything except for newline.  square brackets will be treated like character classes, and so on through the entire list of regexp pattern tokens. quotemeta or \Q and \E are not just quirks providing a lucky solution, this sort of situation is on the short list of reasons for quotemeta and \Q\E to exist.
In perldoc -f quotemeta you'll read, "quotemeta (and \Q ... \E ) are useful when interpolating strings into regular expressions, because by default an interpolated variable will be considered a mini-regular expression. I wouldn't call it an anomaly, it's just how Perl is designed.
perlop contains a good crash course on how interpolation works. The sections to look for start with Quote and Quote-like Operators, and continue through the end of "Gory details of parsing quoted constructs".
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