my $bufsize = 256 * 1024;
sysopen my $in1, 'somefile', O_RDONLY;
sysopen my $in2, 'someother', O_RDONLY;
sysopen my $out, 'outputfile', O_WRONLY|O_CREAT|O_TRUNC;

binmode($in1);  # Is this necessary?
binmode($in2);
binmode($out);

while (sysread($in1, my $buf1, $bufsize) && 
    sysread($in2, my $buf2, $bufsize)) {

        my $data = <CONCAT $buf1 & $buf2>; # HOW?
        syswrite $out, $data, 2 * $bufsize;
}