This will find a case-insensitive "sTRinG" followed by whitespace, even if the string is at the end of the line:

#!/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
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This prints:

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