$ man perlop
:
:
               If "/" is the delimiter then the initial "m" is optional.  With
               the "m" you can use any pair of non-whitespace (ASCII)
               characters as delimiters.  This is particularly useful for
               matching path names that contain "/", to avoid LTS (leaning
               toothpick syndrome).  If "?" is the delimiter, then a match-
               only-once rule applies, described in "m?PATTERN?" below.  If
               "'" is the delimiter, no interpolation is performed on the
               PATTERN.  When using a character valid in an identifier,
               whitespace is required after the "m".
:
:
       m?PATTERN?msixpodualgc
       ?PATTERN?msixpodualgc
               This is just like the "m/PATTERN/" search, except that it
               matches only once between calls to the reset() operator.  This
               is a useful optimization when you want to see only the first
               occurrence of something in each file of a set of files, for
               instance.  Only "m??"  patterns local to the current package
               are reset.