#!/usr/bin/perl -w
use strict;

my $count;
my $word = "this";
while (<DATA>)
{
   my @matches = /\b\Q$word\E\b/gi;
   $count += @matches;
   #
   # or just do this:
   # $count+= () = /\b\Q$word\E\b/gi;
}

print "$count"; #prints 3
# note that the last "thisthis" on line 1
# doesn't count! Also note that multiple "this" on
# same line are counted.


__DATA__
this is the text that this thisthis
abd
this