my $in_file = "bill";
my $out_file = "far2";
open IF, "$in_file" or die $!;
open OF, ">$out_file" or die $!;
while(<IF>) {
    chomp;
    print OF $_;
    if($. == 4012) {
        print OF "... test"; 
    } 
    print OF "\n"; 
} 
print "Done\n";
close(IF);
close(OF);