my %visited   = map { $_ => undef } (1 .. 4);
my @unvisited = (1 .. 9);

while ( @unvisited ) {
    my $router = shift @unvisited;
    next if exists $visited{$router};
    visit ( $router, \@unvisited );   # Adds routers to @unvisited
    $visited{$router} = undef;
}