$twig->parsefile($xml);
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$twig->parse($xml);
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In other words; does your web server log indicate any problems. Because your file(s) don't have the correct Read, Write, or in the case of Perl scripts, Execute bits set (attributes). This is probably the most common case for HTTP-500 errors. If you're on a *NIX OS, chmod, and chown may provide further clues.

Best wishes.

--Chris

There may be other problems but here are the ones that I saw.

In twig_client_parse, you are reading the contents of three XML files and concatenating them all into one string. That string is then sent in a POST to twig_rcvr_server.cgi where it is then passed to XML::Twig's parsefile method but parsefile expects a filename. You could use parse instead however it doesn't like concatenated files. So in twig_client_parse, you should process each file in turn. Also, in the customer and supplier subroutines of twig_rcvr_server.cgi you are trying to use $player again instead of $customer and $supplier respectively.

Here are my untested recommendations. I have cleaned up your code ever so slightly but there is still room for improvement.

# twig_client_parse
use strict;
use warnings;
use LWP::UserAgent;

my $ua = LWP::UserAgent->new;
my $server_endpoint = "http://localhost/cgi-bin/RnD/twig/twig_rcvr_ser
+ver.cgi";

# set custom HTTP Request header fields
my $xml;
my $req = HTTP::Request->new(POST => $server_endpoint);
$req->header('content-type' => 'text/xml');

my @files = glob( "*.xml" );
#print "@files\n";
for my $test ( @files )
{
    #print "this element: $test \n ";
    my $xml = do { local( @ARGV, $/ ) = $test; <> };
    $req->content( $xml );
    my $resp = $ua->request( $req );
    if ( $resp->is_success )
    {
        my $message = $resp->decoded_content;
        print "Received reply:\n $message\n";
    }
    else
    {
        print "HTTP POST error code: ", $resp->code, "\n";
        print "HTTP POST error message: ", $resp->message, "\n";
    }
}
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# twig_rcvr_server.cgi
use strict;
use warnings;
use CGI;
use DBI;
use XML::Twig;
use CGI::Carp 'fatalsToBrowser';

my $cgi = CGI->new;
print $cgi->header(-type => "text/xml", -charset => "utf-8");

my $xml = $cgi->param("POSTDATA");
print "the data $xml is received";

my $username = "abcxyz";
my $password = "xyzabc";
my $dsn = "dbi:mysql:twig:127.0.0.1";
my $dbh = DBI->connect($dsn,$username,$password) or die "Cannot connec
+t to database: $DBI::errstr";

my $sql1 = 'INSERT INTO twig_test(name,ppg,rpg,apg,g,blk)
        VALUES(?,?,?,?,?,?)';
my $sth1 = $dbh->prepare($sql1);

my $sql2 = 'INSERT INTO customer(first_name,last_name,dob,email)
        VALUES(?,?,?,?)';
my $sth2 = $dbh->prepare($sql2);

my $sql3 = 'INSERT INTO supplier(name,address,tel_no)
        VALUES(?,?,?)';
my $sth3 = $dbh->prepare($sql3);

#test
print "print successfully connected to the database" if ( $dbh );

my $twig = new XML::Twig(
    twig_handlers => {
        player      => \&player,
        customer    => \&customer,
        supplier    => \&supplier,
    }
);

$twig->parse( $xml );
#$twig->print;

sub player
{
    my ( $twig, $player ) = @_;

    my @f = (
        $player->field('name'),
        $player->field('ppg'),
        $player->field('rpg'),
        $player->field('apg'),
        $player->field('g'),
        $player->field('blk'),
    );

    print $f[0], "\n";
    print $f[1], "\n";
    print $f[2], "\n";
    print $f[3], "\n";
    print $f[4], "\n";
    print $f[5], "\n";
    #print "\n";

    $sth1->execute( @f ) or die $DBI::errstr;
}

sub customer
{
    my ( $twig, $customer ) = @_;

    my @f = (
        $customer->field('first_name'),
        $customer->field('last_name'),
        $customer->field('dob'),
        $customer->field('email'),
    );

    print $f[0], "\n";
    print $f[1], "\n";
    print $f[2], "\n";
    print $f[3], "\n";
    #print "\n";

    $sth2->execute( @f ) or die $DBI::errstr;
}


sub supplier
{
    my ( $twig, $supplier ) = @_;

    my @f = (
        $supplier->field('name'),
        $supplier->field('address'),
        $supplier->field('tel-no'),
    );

    print $f[0], "\n";
    print $f[1], "\n";
    print $f[2], "\n";
    #print "\n";

    $sth3->execute( @f ) or die $DBI::errstr;
}
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Update: Replaced semicolons with commas in @f in the player and customer subroutines. Oops.