#!/usr/bin/perl
use warnings;
use strict;

my $filename = "stable.tar.gz"; # it's the latest Perl source code

die "filename not found\n" unless -f $filename;

my $size = -s $filename;

my $total_read = 0;

open F, "< $filename"
        or die "can't open $filename\n";

my $bufsize = 2_000_000;
my $buffer;

while ( my $read = sysread(F , $buffer , $bufsize,  ) )
{
    printf "Read %8u bytes\n", $read;
    # do something with $buffer
    $total_read += $read;
}

print "initial size: $size\n";
print "total read:   $total_read\n";
__END__
output:
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read   254916 bytes
initial size: 12254916
total read:   12254916