What I cannot find in the on-line docs

Its an operator thing, not a regex thing --- from perlop under "Binding Operators":

...
       If the right argument is an expression rather than a
       search pattern, substitution, or transliteration, it is
       interpreted as a search pattern at run time.  This can be
       less efficient than an explicit search, because the pat-
       tern must be compiled every time the expression is evalu-
       ated.
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This can sometimes cause problems for newcomers, especially when they use split with a double-quoted string as the split pattern (as seems to happen with undue frequency) and have an escaped metacharacter in the pattern:

    $_ = 'this has a | pipe';
    @a = split /\|/;           # good
    print join(":", @a),"\n"; 
    @a = split "\|";           # oops
    print join(":", @a),"\n";
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In the second case, the double-quoted string is first evaluated and the *resulting* string (sans backslash) is then used as the pattern in the regex.