try creating a tied hash in the subroutine, then set the hashref passed in to that hash:

sub {
 my ($hashref, $key) = @_;
 my %hash;
 tie %hash, ....;
 $hashref->{$key} = \%hash;
}
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Actually, though, I'd think about using a database, starting with maybe DBD::RAM, then on to a indexable database.