Very cool. But can I make one small suggestion? Use this function for length:

sub _length{
  my $a = shift;
  my $dim = shift;
  if ( ref( $a ) =~ /ARRAY/ ){
    print "\t" x $dim;
    print "dim $dim -> ".(scalar @$a)." elements";
    print "\n" if $dim == 0;
    print "\n";
    _length( $_, $dim + 1) for ( @$a );
  }
}
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It's exactly the same logic, but prints newlines a little differently. Otherwise, you don't get the nice formatting for arrays with four or more dimensions.