$key =~ m/^-?\d+$/

For DiveRef(), if $ref is undefined, then it is autovivified into a reference to anonymous array. If $ref can act as a reference to an array, then $ref= $ref->[$key] is attempted. If $key is larger than $#$ref, then @$ref will grow. If $key is less than -@$ref, then Perl will die.

If $ref cannot act as a reference to an array, then $ref might be used as a reference to a hash as described further down.

a reference to an array

If $key can be used as a reference to an array, then it means that you might expect $ref to be a reference to a subroutine.

If UNIVERSAL::isa( $ref, 'CODE' ) is true, then $ref->( @$key ) is attempted.

If $key isn't the last value in @ListOfKeys and the next value is undefined, then &$ref is called in a scalar context and $ref is set to refer to the scalar value returned.

Otherwise, &$ref is called in a list context and $ref is set to refer to an anonymous array containing the value(s) returned.

any (defined) string

This means that you might expect $ref to be a reference to a hash.

For Dive(), if $ref can act as a reference to a hash and <exists $ref-{$key} >> is true, then $ref= $ref->{$key}; is run [and this can't fail nor autovivify].

Otherwise, Dive() returns an empty list and DiveError() can tell you where the problem was.

For DiveRef(), $ref= $ref->{$key} is simply attempted. This may autovivify a hash entry or even a new hash. It may also die, for example, if $ref can't be used as a hash reference.

Note that the order of the above items is significant. It represents the order in which cases are tested. So an undefined $key will only be for derefencing a scalar reference and a numeric key will prefer to treat a reference as an array reference.