#!/usr/bin/perl
use strict;
use warnings;

use Data::Dumper;

my $text = '1-20,1,1,1,2,4,5,8,9,3-4';

my $list = _text_to_list( $text );
print Dumper $list;

sub _text_to_list {
    my $text = shift;

    my %numbers;
    my @list = split /,/, $text;

    for (@list) {
        my @range = split /-/;
        $range[1] = defined $range[1] ? $range[1] : $range[0];
        $numbers{$_} = 1 for $range[0] .. $range[1];
    }

    return [sort {$a <=> $b} keys %numbers];
}