open my $tailfh, "tail -F $logfile |" or die "blunze: $!\n";
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tail -F handles re-opening the logfile on inode change and file truncating. To process the gathered lines in chunks, I write them to another file:

my @lines;
while (<$tailfh>) {
    push @lines, $_;
    unless (-f $lockfile) {
        open my $chunkfh, '>>', $chunkfile or die "more blunze: $!\n";
        $| = 1; # or use IO::File and autoflush
        print $chunkfh @lines;
        close $chunkfh;
        @lines = ();
    }
}
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Then another process can touch the lockfile, process the lines in the chunkfile, truncate it and remove the lockfile. The line gathering process then flushes its @lines.

update: if you're wondering what blunze is - that's black pudding (or blood sausage)

update 2: moved the open/close inside the loop, changed the open mode to append.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}