my @visited   = (1 .. 5);
my %unvisited = map { $_ => undef } 1 .. 9;

while ( %unvisited ) {
    delete @{unvisited}{@visited};
    for ( keys %unvisited ) {
        visit( $_, \%unvisited );    # Adds routers to %unvisited
        push @visited , $_;
    }
}