Ok, then you don't want to completely delete your instance vars. You want to reset the values of those vars to empty:

sub reset_me {
  my $self = shift;
  $self->{known_nicks} = {};
  $self->{known_channelnames} = {};
}
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In fact, you can use that code during your initialization as well. Then when you add a new instance var, you'll have a clean place to initialize it for everyone.

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update: And as an afterthought, this will also permit your class to be reused for subclasses more directly, since you won't be accidentally clearing out variables that your subclasses need to retain.