I'm on Linux, but on my system there is no ttyS or ttyUSB. I tried your script and got the same warnings; however, if you use /dev/tty1, /dev/tty2, etc., then everything comes together. For example, 
#!/usr/bin/perl -slw

use strict;
$|=1;

BEGIN {
    eval "use Device::SerialPort qw( :PARAM ); 1;"; 
    die "$@\n" if ($@);
}

my $port = $ARGV;
foreach $port(@ARGV) {
    my $ob = Device::SerialPort->new($port, 1)
    or die "Could not open $port: $!";
    sleep 1;
    print "$port is open";
    undef $ob;
}

[download]
I used undef $ob rather than $ob->close. It just worked better.

In reply to Re: Supressing error messages on serial port open in Linux by Khen1950fx
in thread Supressing error messages on serial port open in Linux by pashanoid

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