if someone can tell me in how to use delete in a void context, that would also be helpful.

Void context means you are not asking for any return values:

delete $Registry->{$key};              # void context
$res = delete $Registry->{$key};       # scalar context
if (delete $Registry->{$key}) { ... }  # also scalar context
@results = delete $Registry->{$key};   # list context
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Update: 'array' context should be 'list' context (thanks ikegami.)