As has already been pointed out, you can not do that. But in the code you provided, you would not want to. Consider the case when $item2 is undef and $item3 is defined, you'd be selecting from foo where item3 = ''. Not what you want to do.

  my (@where, @params );
  push ( @where, 'item1 = ?' );
  push ( @params, $item1 );
  if ( defined( $item2 ) ) {
     push( @where, 'item2 = ?' );
     push( @params, $item2 );
  }
  if ( defined( $item3 ) ) {
     push( @where, 'item3 = ?' );
     push( @params, $item3 );
  }
  my $script = sprintf("SELECT foo FROM bar WHERE %s",
                       join ( ' AND ', @WHERE ) );

  ...
  $sql->execute( @params );
[download]

This code is a bit more verbose than necessary. It could be cleaned up by using a configuration hash and looping through each of the items you are searching on rather than doing them linearly, but I think its good enough to demonstrate one way of addressing the issue.