Which characters can be used as delimiters for a Perl regular expression?
With the m you can use any pair of non-whitespace characters as delimiters.
So anything goes, except whitespace. The full paragraph for this is:
If "/" is the delimiter then the initial m is optional. With the m you can use any pair of non-whitespace 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 delimiter, no interpolation is performed on the PATTERN. When using a character valid in an identifier, whitespace is required after the m.