@GrandFather I would rather not combine my scripts, as they each have very different purposes, and the two scripts are also used independently on other machines. I think that creating a daemon is worth a try. As I have no formal IT training, this is going to be an interesting learning experience.

As to the common module with a lock file, I am still stuck with how to have two scripts accessing a common module. I have now tried running a script that has package functions and calls the two test scripts. Pretty messy to put it mildly, but is was just an experiment.

#!/usr/bin/perl

package Hvac::i2cAccess;

use strict;
use HiPi::Device::I2C;
use Exporter qw( import );

our @EXPORT_OK = qw( i2c_test1 i2c_test2 );

my $copyit = "Empty";

# call test scripts
# test1 just gets the $copyit value
# test2 sends an i2c object and gets the voltage value in return
#  the voltage value is also copied into $copyit

do "/home/huw/cron_scripts/test1.pl";
print "A\n";
do "/home/huw/cron_scripts/test2.pl";
print "B\n";
do "/home/huw/cron_scripts/test1.pl";
print "C\n";

while( 1 ){
    sleep 1;
}

exit 0;

# ********************************************************* #
sub i2c_test2 {
    # i2c object passed from test2.pl
    my $obj = $_[0];

    # read 2 bytes of data from bus voltage register (address 0x02)
    my @bvr;
    eval{
        @bvr = $obj->bus_read( 0x02, 2 );
        1;
    }
    or do {
         $bvr[0] = 0;
         $bvr[1] = 0;
    };

    # result in upper 13 bits - big-endian order
    my $busV = pack 'C2', $bvr[0], $bvr[1];
    $busV = ( unpack 'S>', $busV ) >> 3;

    # scale bus voltage register value
    my $bv = 16;
    my $bvScale = 4000;
    $busV = $bv / $bvScale * $busV;

    $copyit = $busV;
    return $copyit;
}

# ********************************************************* #
sub i2c_test1 {
    print "Test 1 subroutine\n";
    return "Copy $copyit";
}

# ********************************************************* #
# must end modules with a 'true' value
1;
[download]

but I don't get past the first 'do'. I presume that 'do' is waiting for the script it calls to end and return a value. 'A' is never printed

The module code without the 'calls' to the two test scripts was used to test the common module approach, but as I said, it looks like each test script invokes a separate copy of the module.