perldoc perlsub states: 
    sub maybeset {
             my($key, $value) = @_;
             $Foo{$key} = $value unless $Foo{$key};
    }

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Because the assignment copies the values, this also has the effect of turning call-by-reference into call-by-value. Otherwise a function is free to do in-place modifications of "@_" and change its caller's values.

But if this is the case, then how come 

sub XXX {
   my ($self) = @_;

   $self->{age} = '17';
}

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actually updates the anonymous hashref that was passed in? Supposedly the assignment changed call by reference to call by value and hence any edits to $self should not have affected the caller.

I do this all the time and it just struck me that it shouldn't work.

In reply to How does my $self = (@_) actually work? by princepawn

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