use POSIX 'ceil';
my $buffer_size = 64 * 1024;
my $bytes_wanted = ceil($size / 4);

sub copy {
	my ($in, $out, $bytes) = @_;
	my ($buffer, $bytes_read);

	$bytes_read = sysread($in, $buffer, $bytes);
	print $out $buffer;

	return $bytes_read;
}

open my $in, '<', $infn or die $!;
for my $outfn (1..4) {
	open my $out, '>', $prefix . $outfn or die $!;
	my $bytes = 0;
	while ($bytes + $buffer_size < $bytes_wanted) {
		$bytes += copy($in, $out, $buffer_size);
	}
	copy($in, $out, $bytes_wanted - $bytes);
	close $out;
}
close $in;