DBIx::Class makes this relatively easy, as you can pass additional parameters to a resultset and get back a resultset that will produce a query that matches all of the things you've searched for so far, so you could do something like this:

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";
}
[download]