To be precise (citing from perlop manpage):

• Whitespace is not a valid delimiter, and there are additional semantics associated with single quotes:

  s/PATTERN/REPLACEMENT/egimosx
   
   [ ... snip ...]
  
                 Any non-alphanumeric, non-whitespace delimiter may
                 replace the slashes.  If single quotes are used,
                 no interpretation is done on the replacement
                 string (the /e modifier overrides this, however).
                 Unlike Perl 4, Perl 5 treats backticks as normal
                 delimiters; the replacement text is not evaluated
                 as a command.  If the PATTERN is delimited by
                 bracketing quotes, the REPLACEMENT has its own
                 pair of quotes, which may or may not be bracketing
                 quotes, e.g., s(foo)(bar) or s<foo>/bar/.  A /e
                 will cause the replacement portion to be
                 interpreted as a full-fledged Perl expression and
                 eval()ed right then and there.  It is, however,
                 syntax checked at compile-time.
  [download]

• with m//, you also want to watch out not to use '?' as the delimiter, as this has a different meaning:

     ?PATTERN?
                 This is just like the /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 ??  patterns local to the current
                 package are reset.
  
  [ ... snip ...]
  
         m/PATTERN/cgimosx
  
         /PATTERN/cgimosx
  
  [ ... snip ...]
  
                 If "/" is the delimiter then the initial m is
                 optional.  With the m you can use any pair of non-
                 alphanumeric, non-whitespace characters as
                 delimiters. This is particularly useful for
                 matching Unix 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
                 variable interpolation is performed on the
                 PATTERN.
  [download]

  This is all explained in perldoc perlop (there is no "perlops" manpage).

  Christian Lemburg
  Brainbench MVP for Perl
  http://www.brainbench.com

  Comment on Re: Re: quite weird regexp Select or Download Code