The problem with using  (?:^|\s) and  (?:\s|$) as delimiter patterns is that  \s in the middle of a string requires and consumes a whitespace character. If only a single whitespace character separates patterns that are intended to match, some matches will be missed:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $title = 'C C C C++ C++ C++ .NET .NET .NET';
;;
for my $kw (qw(.NET C C++)) {
  my $count = () = $title =~ m{ (?:^|\s) \Q$kw\E (?:\s|$) }xmsig;
   print qq{'$kw' $count};
   }
"
'.NET' 2
'C' 2
'C++' 2
[download]