If you know that $allocated_to will never contain "0" (a zero), then you can simplify the code to :

if ($allocated_to) {
  push @where_clause, "UPPER(ALLOCATED_TO) LIKE UPPER('%$allocated_to%
+')";
  push @entries, 'ALLOCATED_TO';
}
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(the % don't need to be escaped in Perl strings) - possibly you might want to collapse both cases into a third subroutine like the following :

sub add_if_defined {
  my ($columnname,$queryvar) = @_;

  die "add_if_defined: First parameter (columnname) may not be empty (
+maybe you supplied columnname and queryvar in the wrong order ?)" 
    unless $columnname;

  if (defined($queryvar) && $queryvar ne '') {
   push @where_clause,"UPPER($columname) LIKE UPPER('%$queryvar%')";
   push @entries, $columnname;
  }
};
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  add_if_defined("ALLOCATED_TO",$allocated_to);
  add_if_defined("CONTACT_PERSON",$contact_person);
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This might make your code more readable, but it will also hide the actual side effects in the subroutine.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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