local $. in the <DATA>-block has no usage. What will perl do? it will copy the current value of $. to a temp variable, and reset it after the block.

It is not this block that it is messing up the results of $., it is the seeky-sub. What you could do is add a local $. in sub seeky, then you get the results you expect.

Since this means that at the start of the block it will copy $. to another variable, seek will set $. to another alias, and at the end of the block the value of $. is restored (by using the copy it made).

Update: another solution with which you would get the same result is to create a new block in your code, and localising $. in there. Something like:

while (<DATA>) {
    chomp;
    my $line = $.;
    warn "Pre match: Line $. == $line\n";
    next if /match/;
    {
      local $.;
      seeky();
    }
    warn "Postmatch: Line $. != $line\n";
}
[download]