use strict;
use warnings;

my $fname = shift or die "usage: $0 fname\n";
open(my $fh, '<', $fname) or die "error: open '$fname': $!";

my $chunksize = 4;
my $chunk = "";
while ( my $line = <$fh> ) {
    $chunk .= $line;
    next if $. % $chunksize;
    print "---chunk---\n$chunk";
    $chunk = "";
}
close $fh;
if (length $chunk) {
    print "---chunk (leftover)---\n$chunk";
}