...
       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.

##</code><code>##

    $_ = 'this has a | pipe';
    @a = split /\|/;           # good
    print join(":", @a),"\n"; 
    @a = split "\|";           # oops
    print join(":", @a),"\n";