Noting specifically that you have asked to match for the exact string "String" (rather than a case-insensitive match), this should work:

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

my $toFind="String";

for (<DATA>){
    print if /$toFind(?!\.[oc])\b/;
}

__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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