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- Let's say you have a perl-style accessor
$self->foo(), which is used to access the value$self->{foo}. $self->{foo} is currently eq'bar'.- A caller comes along with a variable $, and wants to set the foo value to whatever's in $. They therefore call
$self->foo($_). - The bug: if $_ is undef, that means that
$self->foo(undef)is called. In a perl-style accessor, that is considered a 'get' operation instead of a 'set', so after that call, $self->{foo} is still set to'bar'.
In other words, it's impossible to use a perl-style accessor to set a value to 'undef', and it's easy to accidentally perform a no-op instead of a set. This has bitten us in the past.
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