#!/usr/bin/env perl
use warnings;
use strict;

my $toFind = 'String';
while (<DATA>) {
    print if /\b$toFind\s/i;
}

__DATA__
this is a string
strings are us
String.c is a file
so string.o is too
I like a good STRING now and again

##</code><code>##

this is a string
I like a good STRING now and again