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 , $_;
    }
}

##</code><code>##

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;
}