You haven't shown your code but it's a fair guess that you haven't used __SUB__ like the documentation tells you to do:

my sub baz {
    __SUB__->();    # calls itself
}
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package MyPairList;
use strict;
use warnings;
use feature 'state';
use Data::Dumper;
use List::Util qw(all none);
use Exporter 'import';
our @EXPORT = qw(pair_list_parse);

sub pair_list_parse {
my $string = shift;
my sub get_tokens {
    my $string = shift;
    my @tokens = $string =~ /([()]|[^()\s]+)/g;
}
my @tokens = get_tokens $string;

my sub parse_list {
    state @tokens = @_;
    my @list;
    while (my $token = shift @tokens) {
        if ($token eq '(') {
            push @list, __SUB__->();
        } elsif ($token eq ')') {
            return \@list;
        } else {
            push @list, $token;
        }
    }
    return \@list;
}

my $list = parse_list @tokens;

my sub parse_nested_list {
    my ($list) = @_;
    my %hash;
    if (none {ref $_} @$list) {
      if (@$list == 1) {
        return $list->[0];
      } elsif (@$list == 2) {
        return { $list->[0] => $list->[1] };
      } else {
        return $list
      }
    } elsif (all {ref $_} @$list) {
        return [ map { __SUB__->($_) } @$list ];
    } else {
        my ($key, $ref, @remain) = @$list;
        die 'wrong key ', Dumper $key if ref $key;
        die 'wrong value ', Dumper $ref unless ref $ref;
        die 'redundant value', Dumper \@remain if @remain;
        $hash{$key} = __SUB__->($ref);
    }
    return \%hash;
}
return parse_nested_list($list)
}
1;
[download]

Last time I just add my to two subs do recusion, and die with 'undeined sub', now I replace it with __SUB__, still 'Undefined subroutine &MyPairList::__SUB__ called at /usr/local/bin/MyPairList.pm line 23, <> line 1.' It did not work.