The error is from using warnings not strict ;)

I'm still a little confused about what you mean. Are you just saying that the first line of a file is "line 1" rather than "line 0" as someone might expect because there are 0 based arrays? Yeah totally, that is correct. Your use of a special variable confused me :) The rest of this node is just a digression and can be ignored if you want :D

I don't think it's really fair to say $. has anything to do with the return value of the .. operator. As I understand it $. contains data from a memory location associated with the last read file handle, and is incremented every time you use <>. Since $. is not just a copy of the line number, but actually is the data stored you can create things like the following ugliness:

---

my $output='$std | dot |$data| dot '."\n";

while(<>) {
    my $std = $_;
    my $std_dot = $.;

    my $data = <DATA>;
    my $data_dot = $.++;

    chomp $std;
    chomp $data;

    last if $std eq '.';

    $output.=sprintf "%4s |%4s |%4s |%4s\n",
        $std, $std_dot, $data, $data_dot;
}

print $output;
__DATA__
a
b
c
d
e
[download]

---

And this has the following input/output:

---

a
b
c
d
e
.
$std | dot |$data| dot
   a |   1 |   a |   1
   b |   2 |   b |   3
   c |   3 |   c |   5
   d |   4 |   d |   7
   e |   5 |   e |   9
[download]

---

I highly doubt that would be useful, but it's interesting to point out how $. is saved. Now, to incorperate the .. operator:

---

my $output='$std | dot | $r1 |$data| dot | $r2 '."\n";

while(<>) {
    my $std = $_;
    my $std_dot = $.;

    my $data = <DATA>;
    my $data_dot = $.++;

    chomp $std;
    chomp $data;
    my ($r1, $r2);

    if(
        ($r1 = $data =~ /a/ .. $data =~ /e/) &&
        ($r2 = $std =~ /a/ .. $std =~ /e/)
    ) {
        $output.=sprintf "%4s |%4s |%4s |%4s |%4s |%4s \n",
            $std, $std_dot, $r1, $data, $data_dot, $r2;
    } else {
        last;
    }
}

print $output;
__DATA__
a
b
c
d
e
[download]

---

This takes/yields the following:

---

a
b
c
d
e
f
$std | dot | $r1 |$data| dot | $r2
   a |   1 |   1 |   a |   1 |   1
   b |   2 |   2 |   b |   3 |   2
   c |   3 |   3 |   c |   5 |   3
   d |   4 |   4 |   d |   7 |   4
   e |   5 | 5E0 |   e |   9 | 5E0
[download]

---

It seems pretty clear that the .. operator uses some kind of internal counter. I know, I'm totally abusing the $. variable in these examples. My question is where does the value of $. go if you use it without the <> operator.