Extremely silly, but amusingly short :)

#!/usr/bin/perl -wl
use strict;

my @a=0..1e3;       # Set higher than any reasonable message number
my @list = ("1, 3, 5-7", "1-5, 2, 4", "1-3, 2-5");

for (@list){
    my %h;
    s/-/../g;
    $h{$_}++ for @a[(eval $_)];
    print join " ", sort {$a<=>$b} keys %h;
}
[download]

Output:

1 3 5 6 7
1 2 3 4 5
1 2 3 4 5
[download]

This relies on Perl interpreting your format (once dashes are replaced by double dots) as array subscripts.

Set::IntSpan?

Probably you'll get better advice but this was a fun and straightforward little problem so-

package MyRanger;
use strict;
use Carp;

sub new {
    my $caller = shift;
    my $self = bless {}, $caller;
    $self->{_range_string} = shift || croak "You must provide a range"
+;
    for my $entry ( split /\s*,\s*/, $self->{_range_string} )
    {
        if ( $entry =~ /\A\d+\z/ )
        {
            push @{$self->{_raw_range}}, $entry;
        }
        elsif ( $entry =~ /\A(\d+)-(\d+)\z/ )
        {
            carp "Range '$entry' is impossible, skipping"
                and next if $1 > $2;
            $entry =~ s/-/../;
            push @{$self->{_raw_range}}, eval $entry;
        }
        else
        {
            croak "Illegal range entry '$entry' given";
        }
    }
    @{$self->{_range}} = sort { $a <=> $b } keys %{ { map {$_ => 1} @{
+$self->{_raw_range}} } };
    @{$self->{_range}} || croak "No range was parseable in ", $self->{
+_range_string};
    $self->{_pos} = 0;
    $self;
}

sub next {
    my $self = shift;
    $self->{_range}->[$self->{_pos}++];
}

1;

my @range_strings = (
                     "1, 3, 5-7",
                     "1-5, 2, 4, 10, 100",
                     "1-3, 2-5",
                     "12-10",
                     );

for my $string ( @range_strings )
{
    print "MyRanger->new with '$string'\n";
    my $ranger = MyRanger->new($string);

    while ( my $item = $ranger->next() ) {
        print "\tNext item is: $item\n";
    }
}
[download]

Extra stuff like-

sub size {
    my $self = shift;
    scalar @{$self->{_range}};
}
[download]

...d'oh, left up to you. :)

Fun!

sub strings_to_one_span {
  my %numbers;
  @numbers{ map { (/(-?\d+)-(-?\d+)/ or /((-?\d+))/) ? ($1 .. $2) : ()
+ } split /\s*,\s*/ } = ()
    for @_;
  sort { $a <=> $b } keys %numbers
}
[download]

[]s, HTH, Massa (κς,πμ,πλ)

You didn't say what you wanted this object to do...

If you want it to store the range list, and then be used as a sort of iterator, then you might want to split the work in two:

• during the initialisation of the object, validate the ranges, remove all duplicates and overlaps and store the resulting list of ranges in ascending order.

• implement a "next" method for use:

    while (my $num = $range->next) { }
  [download]

• housekeeping to reset the iterator, test its state etc.