If "/" is the delimiter then the initial "m" is optional.  With
               the "m" you can use any pair of non-alphanumeric, non-white-
               space characters as delimiters.  This is particularly useful
               for matching path names that contain "/", to avoid LTS (leaning
               toothpick syndrome).  If "?" is the delimiter, then the match-
               only-once rule of "?PATTERN?" applies.  If "'" is the delim-
               iter, no interpolation is performed on the PATTERN.
...
               If the delimiter chosen is a single quote, no interpolation is
               done on either the PATTERN or the REPLACEMENT.  Otherwise, if
               the PATTERN contains a $ that looks like a variable rather than
               an end-of-string test, the variable will be interpolated into
               the pattern at run-time.  If you want the pattern compiled only
               once the first time the variable is interpolated, use the "/o"
               option.  If the pattern evaluates to the empty string, the last
               successfully executed regular expression is used instead.  See
               perlre for further explanation on these.  See perllocale for
               discussion of additional considerations that apply when "use
               locale" is in effect.