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

my %visited   = map { $_ => undef } (1 .. 4);
my @unvisited = (1 .. 9);
...
    visit ( $router, \@unvisited );   # Adds routers to @unvisited
    $visited{$router} = undef;
}