what does the eval do ?

If a registry key contains values, then keys will return the names of those values. But $key->{$aValueName} will not give you back a hash reference so the %{$key->{$name}} would fail with "Can't coerce array into hash" or something.

sub deleteReg {
    my( $key, $name )= @_;
    for(  eval { keys %{$key->{$name}} }  ) {
        deleteReg( $key, "$name\\$_" );
    }
    delete $key->{$name};
}
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The eval will return what keys returns if $name is a subkey name and will trap the die and return and empty list if $name is a value name. So for( eval ... ) will silently skip values.

Also after executing this code i end up with a default value in the hive and can't find a way to delete it ?!

You can't have a registry key without an empty default value. That is just the way the registry is.