You can improve this by using placeholders.

use DBI;

   $DSN  = "dbi:Oracle:host=localhost;sid=essinv;port=1521";
   $user = "username";
   $pw   = "password";
   $dbh  = DBI->connect($DSN,$user,$pw, { RaiseError => 1, AutoCommit 
+=> 1})
       || die "Cannot connect: $DBI::errstr\n" unless $dbh;


   print header ();

   eval{
     $SQL = "INSERT INTO EMPLOYEE (
                 EMPLOYEE_NUMBER, FIRST_NAME, LAST_NAME, INFORMATION_D
+ATE)
                 VALUES ( ? , ? , ? , SYSDATE)";
     $sth = $dbh->prepare($SQL);
   }; # End of eval

   # Check for errors.
   if($@){
     $dbh->disconnect;
     print "Content-type: text/html\n\n";
     print "An ERROR occurred! $@\n<P>";
     exit;
   } else {
     $sth->execute(param('employee_number'),param('first_name'),param(
+'last_name'));
   } # End of if..else

sub print_output {
   print <<HTML;
   PRINT CONFIRMATION HERE
HTML
}


   # Disconnect from the database
   $sth->finish;
   $dbh->disconnect;
[download]

As for finding if it exists first this may or may not be needed depending on the database structure. How the item is created/maintained will alter my idea of what the best solution would be.