I got it "simpler" before changing to this version, but it didn't work either. Here's the modified version:

    use Strict;
    use IO::Socket;
    use IO::Select;

    my $COMport=33;
    
    #open Port or die if blocked / wrong port
    if (!open( PORT, "+>\\\\.\\COM$COMport" ))
        {  
            die ("Can't open COM$COMport");
        }
        
    PORT->autoflush(1);
    PORT->flush();

    print "\nEnter AT command:";
    my $ATcmd=<STDIN>;
    chomp($ATcmd);         # remove CRLF
    $ATcmd =~ s/^\s+//;        # remove leading space
    $ATcmd =~ s/\s+$//;            # remove trailing space
    print "Sending: '$ATcmd'\n";
    print PORT $ATcmd . "\r\n";
    sleep(2);

        #Reading the response
        print "reading\n";
        while (1)
        {   
            my $ret=<PORT>;
            print $ret;
            if ($ret=~/^OK/ || $ret=~/^ERROR/) { last}
        }

    close(PORT);
    print "done!";
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