chomp ( my @id = <ID_LIST> );
my %ids;
@id{@id} = (1) x @id;

# dropped the chomp because you added \n back in
while ( my $data = <DB> ) {
    my ($curr_id) = $data =~ /(user\d\d-\d\d\.dept\d\d)/;
    if ( $curr_id and exists $id{ $curr_id } ) {
        print OUT $data;
    }
}