Two suggestions.

1. Change:

   #!C:/Perl64/bin/perl.exe
   use DBI;

   to

   #!C:/Perl64/bin/perl.exe
   use strict;
   use warnings;
   use DBI;

   and;
    

2. Change:

   $sth = $dbh->prepare("SELECT username, password FROM users WHERE username =$username and password=$password");

   to

   my $sqlStatement = "SELECT username, password FROM users WHERE username =$username and password=$password";
   print "Submitting:  $sqlStatement\n";
   $sth = $dbh->prepare($sqlStatement);

   
    

See what the full SQL statement looks like; you are likely to find an error there, since that's what your error message says.

Most SQL requires strings to be encapsulated in single-quotes:  ... and password = 'newpassword';

Dunno if that's your problem, but the above changes should help shed light on what you are getting wrong in the SQL statement.