I was going to answer something like that, but forgot all about it. In the meantime, I've read (once again) some parts of
CGI.pm, and found these interesting regexes:
$toencode =~ s{&}{&}gso;
$toencode =~ s{<}{<}gso;
$toencode =~ s{>}{>}gso;
$toencode =~ s{"}{"}gso;
...
$toencode =~ s{'}{'}gso;
$toencode =~ s{\x8b}{‹}gso;
$toencode =~ s{\x9b}{›}gso;
...
$toencode =~ s{\012}{ }gso;
$toencode =~ s{\015}{ }gso;
That's at least 9 uses of /o with no variable interpolation in the pattern.
But CGI also doesn't use strict, has no spaces after commas, uses C-style for-loops where lists where more efficient and better readable, assumes that the IP address "0" equals "127.0.0.1", has ugly #-laden comments, etc... So is the use of /o really more efficient when no variable interpolation is used, or is this yet another strange thing in CGI.pm. (Excuse me for being so negative about CGI.pm, but having read it, I prefer my own set of functions even more.)
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7220756E7061636B3F202F6D736720
6D6521203A29202D2D204A75657264