This will surround all your hit words with square brackets and show the hit count:

use strict;
use warnings;

my $line = 'This line has a hit here and a hit there.';
my $word = 'hit';
my $count = $line =~ s/\b($word)\b/[$1]/gi;
print "$line\n";
print "$word was found $count times\n";

__END__

This line has a [hit] here and a [hit] there.
hit was found 2 times
[download]

The substitution operator returns the number of substitutions made.