Your applying binmode to the output file, by which time I think the damage has already been done.

What I was suggesting was to apply binmode to the input file. Whilst you don't open this directly, looking at the source of Device::SerialPort, I noticed that the filehandle it use for the serial port is accessable via the object as $obj->{FD}, hence it should be possible to enable the :utf8 IO layer on it using the snippet I included. I've modified your snippet

#!/usr/bin/perl
use warnings;

use Device::SerialPort 0.22;

$LOGDIR    = "/home/neo/telex_lusa/log";
$LOGFILE   = "geiger.log";            
$PORT      = "/dev/ttyS0";            

$ob = Device::SerialPort->new ($PORT) 
   || die "Can't Open $PORT: $!";

## MODIFIED HERE!!
## FD is the key used internally by D::SP for the file handle
## NOTE: The IOLayer is ':utf8' not ':UTF8'!
binmode $ob->{FD}, ':utf8';

$ob->baudrate(2400)   || die "failed setting baudrate";
$ob->parity("none")    || die "failed setting parity";
$ob->databits(8)       || die "failed setting databits";
$ob->stty_icrnl(1)     || die "failed setting convert cr to new line";
$ob->handshake("none") || die "failed setting handshake";

$ob->write_settings    || die "no settings";

open(LOG,">>${LOGDIR}/${LOGFILE}")
||die "can't open smdr file $LOGDIR/$LOGFILE for append: $SUB $!\n";

select(LOG), $| = 1;    # set nonbuffered mode, gets the chars out NOW

open(DEV, "<$PORT") || die "Cannot open $PORT: $_";

while($_ = <DEV>){       
    print LOG $_;
}
undef $ob;
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As I said, I've no way to verify this. For the record and those that come after you, please post a reply stating one way or another whether this works.