You need to take a step back and think about what you're doing, rather than just copying/pasting things and hoping for the best. You've simply printed a second hash, using the string 'username' combined with the hash you got the first time round. Why would you do this? When reading in the user input you want to generate the hash then compare it to the one stored in your text file. MD5, Digest::MD5.

"When reading in the user input you want to generate the hash then compare it to the one stored in your text file."

Thanks for the above explanation...

Now, Let's break this statement..

first "When reading in the user input"

In this case, user inputs USERNAME as "username" and PASSWORD as "SamplePassword".

in my login.cgi file, below 2 lines carry them.

my $var_username = param( "USERNAME" );
my $var_password = param( "PASSWORD" );
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secondly "you want to generate the hash"

now, $var_password is PLAIN text and NAKED. Now, I will have to use "use Digest::MD5;" to encrypt it.

this is the code for it.

my $encrypted_password = Digest::MD5::md5_hex("$var_username" . "$var_
+password");
#print "encrypted_password : $encrypted_password \n"; # This shows the
+ encrypted password.
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Thirdly, "then compare it to the one stored in your text file."

Now, this encrypted password contains the /tmp/password.txt file along with username in this way.

username,c0075ad4e26ec3dee225ccb6387b0b77
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Now, Let's apply it to the code. Now I need $encrypted_password instead of $var_password.

( $var_password is PLAIN and not in /tmp/password.txt file. but $encrypted_password is stored in /tmp/password.txt )

here I am gonna apply it.

 if ( ( $var_username eq $username ) && ( $encrypted_password eq $pass
+word ) ) {
   print "$var_username, $encrypted_password <br>"; # I will remove th
+is in real world
   print "Permission has been granted <br>";
   print "<META HTTP-EQUIV=refresh CONTENT=\"$t;URL=$url\">\n";
   $didmsg=1; 
   last; 
   }
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Now, It works. thanks a LOT. here's the full code. If there are any issues, Pls come back.

my login.cgi

#!/usr/bin/perl
use CGI qw(:standard);
use strict;
use warnings;
use Digest::MD5;

my $var_username = param( "USERNAME" );
my $var_password = param( "PASSWORD" );

my $encrypted_password = Digest::MD5::md5_hex("$var_username" . "$var_
+password");

my $url="http://host.redirectiondomain.com:9999/";
my $t=1; # time until redirect activates

print "Content-Type: text/html; charset=utf-8\n\n";

open ( FILE, "/tmp/password.txt" ) || die "The file could not be opene
+d";

my $didmsg=0;
while ( my $line = <FILE> )
{
   chomp $line;
   ( my $username, my $password ) = split( ",", $line );
   
   if ( ( $var_username eq $username ) && ( $encrypted_password eq $pa
+ssword ) ) {
   print "$var_username, $encrypted_password <br>"; # I will remove th
+is in real world
   print "Permission has been granted <br>";
   print "<META HTTP-EQUIV=refresh CONTENT=\"$t;URL=$url\">\n";
   $didmsg=1; 
   last; 
   }
   
   elsif ( ( $var_username eq $username ) && ( $encrypted_password ne 
+$password ) ) {
   print "$var_username, $encrypted_password <br>"; # I will remove th
+is in real world
   print "You entered an invalid password. <br>";
   print "Access has been denied. <br>";
   $didmsg=1; 
   last; 
   }

}
close( FILE );

   unless($didmsg) {
   print "$var_username, $encrypted_password <br>"; # I will remove th
+is in real world
   print "You entered an invalid username. <br>";
   print "Access has been denied. <br>";
   }
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" If there are any issues, Pls come back."

Remove the part about plain text and my first reply and it's still valid.

See Password verification for the underlying principles.