Scan a very big file, working on some portions? This a good opportunity for mmap. For example:

use File::Map ':all';

my $file = shift;
my $SEP = "\n";        # line/chunk separator
my ($s, $e);

open(my $fh, '<', $file) || die "$!";
map_handle(my $mmap, $fh, '<');

sub process {}

for ($e = 0;; $e += length($SEP)) {
    ($s, $e) = ($e, index($mmap, $SEP, $e));
    print "fragment [$s, $e)\n";
    process($e < 0 ? substr($mmap, $s) : substr($mmap, $s, $e-$s));
    last if $e < 0;
}
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