(1) The second strong recommendation in tr18 is from its section
1.6 Line Boundaries, where it reads:

    It is strongly recommended that there be a regular expression
    meta-character, such as "\R", for matching all line ending
    characters and sequences listed above (e.g. in #1). It would
    thus be shorthand for:

    ( \u000D\u000A | [\u000A\u000B\u000C\u000D\u0085\u2028\u2029] )

(2) Perl has 4 predefined character classes \h \H \v \V for vertical and horizontal whitespace.

\h matches any character considered horizontal whitespace; this includes the space and tab characters and several others listed in the table below.
\H matches any character not considered horizontal whitespace.
\v matches any character considered vertical whitespace; this includes the carriage return and line feed characters (newline) plus several other characters, all listed in the table below.
\V matches any character not considered vertical whitespace.