my $rs = $schema->resultset( 'Users' );

if ( my $year = $cgi->param( 'birth_year' ) ) {
    $rs = $rs->search( birth_year => $year );
}
if ( my $fname = $cgi->param( 'first_name' ) ) {
    $rs = $rs->search( first_name => $fname );
}

while ( my $user = $rs->next ) {
    print "User ".$user->name." matched!\n";
}