PATTERN may contain variables, which will be interpolated (and
               the pattern recompiled) every time the pattern search is evalu-
               ated, except for when the delimiter is a single quote.  (Note
               that $(, $), and $| are not interpolated because they look like
               end-of-string tests.)  If you want such a pattern to be com-
               piled only once, add a "/o" after the trailing delimiter.  This
               avoids expensive run-time recompilations, and is useful when
               the value you are interpolating won't change over the life of
               the script.  However, mentioning "/o" constitutes a promise
               that you won't change the variables in the pattern.  If you
               change them, Perl won't even notice.  See also
               "qr/STRING/imosx".