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for my $i ( 0 .. $#{ $self->{ keys } } ) {
    my $id = $self->{ keys }->[ $i ];
    # ...
}

for my $i ( 0 .. $self->max_index() ) {
    my ( $key, $value ) = $self->at( $i );
    # ...
}

my $iter = $self->iterator();

while( my ( $key, $value ) = $iter->() ) {
    # ...
}