B::Deparse is a great tool for this kind of debugging. Running your snippet with the switch -MO=Deparse gives

if (defined $hash{$key} and do {
    $key
}->hash =~ /\w+/) {
    $hash{$key} = 'Numbers: ' . $hash{$key};
}
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As you see, and as Fletch has already told you, method BLOCK is valid and an indirect method invocation.

lodin