I also already linked the "Basic Debugging Checklist" for you. This list is not for admiration. It contains concrete, actionable points, and you should perform all the actions described there and inspect the results. Most actions are presented together with a rationale of which problem the action tries to diagnose. Maybe you can find out whether one of the actions diagnoses a problem you have.

Corion, below is what I see: The code I run:

#!/usr/bin/perl
##!/comptel/ccacp/elink/common/6.3/perl/bin/perl
use strict;
use warnings;
#use diagnostics;
use CGI;
use CGI::Carp 'fatalsToBrowser';
use Data::Dumper;
print Dumper(\%hash);
print Dumper($ref);

my $sqlfile  = '/var/www/cgi-bin/dual.sql';

my $q = new CGI;
my $connstr = 'id971934/colecsp@ECSP';;
my $msg = qx"sqlplus -s $connstr \@$sqlfile";

print $q->header,$q->start_html,
$q->pre($msg),$q->end_html;

cat /var/www/cgi-bin/dual.sql
select SYSDATE from dual;
exit;
[download]

********************** This script was working properly & was giving proper results(sysdate). But when I introduced "Data::Dumper", and I try to run it from command line I get below messages

[ccacp@el2310 cgi-bin]$ perl monkey.pl
Status: 500
Content-type: text/html

<h1>Software error:</h1>
<pre>Global symbol &quot;%hash&quot; requires explicit package name at
+ monkey.pl line 9.
Global symbol &quot;$ref&quot; requires explicit package name at monke
+y.pl line 10.
Execution of monkey.pl aborted due to compilation errors.
</pre>
<p>
For help, please send mail to this site's webmaster, giving this error
+ message
and the time and date of the error.

</p>
[Fri Mar 18 17:41:22 2016] monkey.pl: Global symbol "%hash" requires e
+xplicit package name at monkey.pl line 9.
[Fri Mar 18 17:41:22 2016] monkey.pl: Global symbol "$ref" requires ex
+plicit package name at monkey.pl line 10.
[Fri Mar 18 17:41:22 2016] monkey.pl: Execution of monkey.pl aborted d
+ue to compilation errors.
[download]

************************** On running from web browser, I see HTTP 500 error with message "the website cant be displayed".

What error did you get from using DBI ?. Try this simple script

#!/usr/bin/perl
use strict;
use warnings;
use CGI;
use CGI::Carp 'fatalsToBrowser';
use DBD::Oracle;

my $q = new CGI;
my $dbh = DBI->connect(
  'dbi:Oracle:host=localhost;sid=DB','USER','PASSWORD', 
  {RaiseError => 1 , PrintError => 0} );     

my ($msg) = $dbh->selectrow_array(
   'SELECT CURRENT_TIMESTAMP FROM DUAL');

print $q->header,$q->start_html,
$q->h2($msg),$q->end_html;
[download]

Hi, What DB, USER & PASSWORD do I need to use? I don't see DB entry in your connect string. Anyways, I adapted USER & PASSWORD info & it threw HTTP 500 error. Regards