use warnings;
use strict;

open(my $fh_1, '>>:raw', 'file1.bin');
open(my $fh_2, '<:raw', 'file2.bin');

my $buffer;
my $bufsize = 32768;
my $copied;

while (1) {
    $copied = read $fh_2, $buffer, $bufsize;
    last if ! $copied;
    print $fh_1 $buffer;
}