So what are the 12 bytes vs the 13 bytes, using the hex editor?

If you're using CRLF as you say, you would get 14 and 16.

1   2   3   4           5   6   7   8   9   10  11  12  13  14 
"a" "b" CR  LF          "_" "_" "D" "A" "T" "A" "_" "_" CR  LF
"a" "b" CR  LF  CR  LF  "_" "_" "D" "A" "T" "A" "_" "_" CR  LF
1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
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None of PerlIO, clib and the OS know anything of __DATA__, so they don't reporting the file positions different after the first __DATA__ is encountered.

I didn't take anything out of context. You said you get a shift of 2 before DATA and a shift of 1 (13-12) after DATA. Yet you say the difference in the file is two characters (CR LF) in both cases. Those statements are contradictory.

Have a second look at my previous post. I did get 14 and 16 with the hex editor.

None of PerlIO, clib and the OS know anything of __DATA__, so they don't reporting the file positions different after the first __DATA__ is encountered.

Special Literals

Text after __DATA__ may be read via the filehandle PACKNAME::DATA , where PACKNAME is the package that was current when the __DATA__ token was encountered. The filehandle is left open pointing to the contents after __DATA__. It is the program's responsibility to close DATA when it is done reading from it.

Correct me if I'm wrong but the way I understand it is Perl has to compile and preprocess the program file. It reads the file until EOF or __END__ or __DATA__. According to the above section DATA is left open pointing to just after __DATA__. That means when you do a "tell DATA" like I did at the beginning of the script there is nothing for tell() to calculate: It just has to spit out the file location. Then, after reading from DATA if you do another tell DATA it has to do the math.

I just ran the script on Knoppix linux and this problem didn't show up. The seek went to the correct location and there were no partial lines printed.

I'm sorry that I couldn't explain about the offsets better. I know what I meant and it makes sense to me. I thought it was an interesting problem that others would be interested in or I wouldn't have posted it.

I've had a splitting headache all day and now that I've had some Tylenol and a snack I'm off to relax a little.

I can confirm the bug (ActivePerl 5.12.1). It doesn't happen with PERLIO=:crlf on a linux machine. (5.12.2, default build config).

Here's a good demonstration:

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

my @data_positions = tell(DATA);
while (<DATA>){
    if (/^__DATA__$/) {
        push @data_positions, tell(DATA);
    }
}

my @fh_positions;
open(my $fh, '<', $0) or die;
while (<$fh>){
    if (/^__DATA__$/) {
        push @fh_positions, tell($fh);
    }
}

print("@data_positions\n");
print("@fh_positions\n");

__DATA__
ab
__DATA__
ab

__DATA__
ab
__DATA__
lotsa junk
nothing
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389 401 414 426
389 403 419 433
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