Here is an old usenet posting which has some good tips in, which you might try. It works on linux.

#!/usr/bin/perl

#the ubiquitous RS-232 Serial Port.  built an interface 
#for my Aware Electronics Geiger Counter (RM-70) that 
#used a Basic Stamp IISX chip set. Every 20 seconds it 
#would spit out an ASCII string of radiation and 
#temperature data. I had hunted around for examples 
#using the Device::SerialPort module and found many. 
#Most were copies of code used to read PBX data. But 
#it wouldn't work. Ouch. Finally, after much research 
#I realized that - by golly - you had to terminate 
#the IO with a new line and not a carriage return. 
#Oddly doing a cat </dev/ttyS0 worked, which faked me 
#out. So I added the line in the tty setup below that 
#converts CR to NL and zap-ity-do-dah it started terminating 
#and sending each read. Long live RS-232. Please send 
#along any suggestions and improvements. This also probably 
#explains why I had failed to get good reads from a 
#cheap-o RS-232 capable DVM a few years ago. 

#This was done in Perl 5.8.0 under RH Linux 9.0. 

#file geiger.pl 
# 
# Author: David Drake 
# Date:   July 18, 2003 
# Requirements: Device::SerialPort 0.22 (from cpan  July 2003) 
# 
# Version: 1.0 
#This script is used to read a serial port to obtain data from a 
#combined Geiger counter and temperature sensor. 
#The Geiger Counter is an Aware Electronics RM-70 unit. Each count 
#maps to one microR per hour. The RM-70 pulse output is sent to a 
#Basic Stamp-IISX microcontroller. The BS2SX accumulates counts for 20
+ 
#seconds and then sends a serial data stream out of its serial port. 
#The data stream goes into the input serial port on the Linux system. 
#This program then tabulates the data to a log file. 

use Device::SerialPort;
use Time::gmtime;

$LOGDIR  = "/home/zentara/perlplay/serial-comm";    # path to data fil
+e 
$LOGFILE = "geiger.log";                      # file name to output to
+ 
$PORT    = "/dev/ttyS1";                      # port to watch 

# 
# 
# Serial Settings 
# 

#make the serial port object 
#note the need to convert carriage returns to new lines to terminate e
+ach 
#read. 

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

$ob->baudrate(9600)    || 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 the logfile, and Port 
# 

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: $_";

# 
# Loop forver, logging data to the log file 
# 

while ( $_ = <DEV> ) {    # print input device to file 
    $gmc = gmctime();
    print LOG $gmc, " ", $_;
}

undef $ob;

#we are done dude
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