my $bufsize= 4096;
my $align= 512;
my $buf= 'x' x ($align+$bufsize);
my $off= unpack( "J", pack "p", $buf ) % $align;
$off= $align - $off
    if  $off;
sysread( $FH, $buf, $bufsize, $off )
    or  ...;
my $data= substr( $buf, $off, $bufsize );