use CGI;
my $cgi = new CGI;

my $user = $cgi->param("username");
my $pass = $cgi->param("password");
my $valid = 0;

# Assuming usernames/passwords in a delimited text file
open(IN, "passwordFile") or die(...);
while(<IN>) {
  if( /^$user\:$pass$/ ) {
    $valid = 1;
    last;
  }
}
close(IN)

if(!$valid) {
  print "Sorry, you may not access this page.";
  exit;
}

#set authentication
#set a cookie here using javascript 
   or
#add hidden fields with login info
print "<input type='hidden' name='pass' value='...'>";
print "<input type='hidden' name='user' value='...'>";


Store usernames/passwords in a file as follows:
johndoe:myPassw0rd
user2:pass2
user3:pass3
etc