use Fcntl;  # for SEEK_SET and SEEK_CUR
$ADDRESS = $RECSIZE * $RECNO;
seek(FH, $ADDRESS, SEEK_SET)  or die "Seeking: $!";
read(FH, $BUFFER, $RECSIZE) == $RECSIZE
                              or die "Reading: $!";
@FIELDS = unpack ($FORMAT, $BUFFER);
#update fields, then
$BUFFER = pack($FORMAT, @FIELDS);
seek(FH, -$RECSIZE, SEEK_CUR) or die "Seeking: $!";
print FH $BUFFER;
close FH                      or die "Closing: $!";