If you pass in a hashref as the first argument, $_[0] refers to aliases that scalar. The lexically scoped copy produces a second hashref to the same external hash.

Some variations:

sub XXX {
    my ($self) = @_;
    $self->{age} = '17';   # does as you say
    $_[0]->{'age'} = '21'; # a miraculous drivers license
    $self = { %{$self} };  # now we have a copy of the innards
    $self->{age} = '17';   # restoration of youth
# the world now thinks age=>21, we have private age=>17
# Youth will be lost when the sub exits
}
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Hope that helps.

After Compline,
Zaxo

Update: Ovid uses 'alias', and it's a better term for it. Less chance of confusion.