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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