turn on debug mode

In below perl code , how can I turn on debug for sendSMTP ??
Any idea ???What is the sue of var $debug ???

Code:

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#!/usr/bin/perl -w

use Socket;
use strict;

my($mailTo)    = '[email protected]';
my($mailServer) = 'mail.hgcbroadband.com';

my($mailFrom)  = '[email protected]';
my($realName)  = "chuikingman";
my($subject)    = 'Test send_mail';
my($body)      = "Test Line One.\nTest Line Two.\n";

$main::SIG{'INT'} = 'closeSocket';

my($proto)      = getprotobyname("tcp")        || 6;
my($port)      = getservbyname("SMTP", "tcp") || 25;
my($serverAddr) = (gethostbyname($mailServer))[4];

die('gethostbyname failed.') unless (defined($serverAddr));

socket(SMTP, AF_INET(), SOCK_STREAM(), $proto)
    or die("socket: $!");

 my $packFormat;
$packFormat = 'S n a4 x8';  # Windows 95, SunOs 4.1+
#$packFormat = 'S n c4 x8';  # SunOs 5.4+ (Solaris 2)

connect(SMTP, pack($packFormat, AF_INET(), $port, $serverAddr))
    or die("connect: $!");

select(SMTP); $| = 1; select(STDOUT);    # use unbuffered i/o.

{
    my($inpBuf) = '';

    recv(SMTP, $inpBuf, 200, 0);
    recv(SMTP, $inpBuf, 200, 0);
}
my $debug = 1 ;
sendSMTP(1, "HELO\n");
sendSMTP(1, "MAIL From: <$mailFrom>\n");
sendSMTP(1, "RCPT To: <$mailTo>\n");
sendSMTP(1, "DATA\n");

send(SMTP, "From: $realName\n", 0);
send(SMTP, "Subject: $subject\n", 0);
send(SMTP, $body, 0);

sendSMTP(1, "\r\n.\r\n");
sendSMTP(1, "QUIT\n");

close(SMTP);

sub closeSocket {    # close smtp socket on error
    close(SMTP);
    die("SMTP socket closed due to SIGINT\n");
}

sub sendSMTP {
    my($debug)  = shift;
    #my($debug)  = 1;
    my($buffer) = @_;

    print STDERR ("> $buffer") if $debug;
    send(SMTP, $buffer, 0);

    recv(SMTP, $buffer, 200, 0);
    print STDERR ("< $buffer") if $debug;

    return( (split(/ /, $buffer))[0] );
}

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