In Perl, you can use the 'word boundary' zero-length pattern \b.  So you
can
do
        s/\bMUST\s*NOT\b/{\\MUSTNOT}/g
and
        s/\bMUST\b/{\\MUST}/g

Alternately, you could do
        s/\bMUST(?: (NOT))?\b/{\\MUST$1}/g
to match and replace them both in one pass.