# untested:
my @urls;
for my $outer ( keys %hash ) {
  for my $inner ( keys %{ $hash{$outer} } ) {
    # maybe check before, if entry "url" exists or is defined ...
    push @urls, $hash{$outer}->{$inner}->{url};
  }
}

##</code><code>##

for my $outer ( keys %hash ) {
    for my $inner ( keys %{ $hash{$outer} } ) {
        push @urls, map { $_->{url} }  @{ $hash{$outer}->{$inner} }
    }
}