You could add a attribute initialization method in the parent package and have the Some::Package::Sub constructor call it along with the parent constructor e.g

## in Some::Package
sub _init_attribs {
   %$fields = %{+pop};
}

## in Some::Package::Sub
my %fields = map { $_ => 1 } qw( field_a anotherone );

sub new {
    my $self = shift;
    $self->_init_attibs(\%fields);
    $self->SUPER::new(@_);
}
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So you can still use the %fields lexical and Some::Package's constructor. How's that?
HTH

_________
broquaint

update: changed code to DWOrsmoMeant