I reduced the code to the minimum to reproduce the problem.

#!/usr/bin/perl 

use strict;
use warnings;
use Device::SerialPort;

my $comm;

sub foo {
  $comm=0;
}

$comm=new Device::SerialPort('/dev/ttyS0') 
 or die "Error opening device\n";
[download]

And this is the error I get:

sini@ordinalfabetix:~$ ./x.pl
        (in cleanup) Can't call method "setcflag" on an undefined valu
+e at /usr/lib/perl5/Device/SerialPort.pm line 482 during global destr
+uction.
sini@ordinalfabetix:~$
[download]

Sub foo is not called anywhere but if I delete it the problem disappear. Same if I delete or replace its content.