the regexp only needs to be compiled once

You gotta read the rest of the text right around there. I'd quote it, but I don't have my camel with me (I'm on a trip). But the perlop manpage says:

               PATTERN may contain variables, which will be interpolat
+ed (and
               the pattern recompiled) every time the pattern search i
+s evalu-
               ated, except for when the delimiter is a single quote. 
+ (Note
               that $(, $), and $| are not interpolated because they l
+ook like
               end-of-string tests.)  If you want such a pattern to be
+ com-
               piled only once, add a "/o" after the trailing delimite
+r.  This
               avoids expensive run-time recompilations, and is useful
+ when
               the value you are interpolating won't change over the l
+ife of
               the script.  However, mentioning "/o" constitutes a pro
+mise
               that you won't change the variables in the pattern.  If
+ you
               change them, Perl won't even notice.  See also
               "qr/STRING/imosx".
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Thus, /o is useful only when there are variables in the pattern. And you had no patterns with variables!