$resp is a reference to a hash, not a hash, so
print $resp->{...
or
print $$resp{...

Also, You seem to not have noticed the [, so
print $resp->{xmlref}{Details}[$i]{ReleaseDate}
or
print $$resp{xmlref}{Details}[$i]{ReleaseDate}

To print release date of all items:

print($_->{ReleaseDate}, $/)
   foreach (@{$$resp{xmlref}{Details}});

[download]

To store the release dates of all items into an array:

@release_dates = map { $_->{ReleaseDate} } @{$$resp{xmlref}{Details}};

[download]

In reply to Re: traversing a list of objects by ikegami
in thread traversing a list of objects by patrickrock

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