my $line = 'This line has a hit here and a hit there.';
my $word = 'hit';

my $count = 0;
my $hilit = '';
while ($line =~ /(.*?)(?:\b($word)\b|\z)/sgi) {
   $hilit .= $1;
   if (defined($2)) {
      ++$count;
      $hilit .= "[$2]";
   }
}

print "$hilit\n";
print "$count occurrences of $word\n";