$ perl -Mstrict -Mwarnings -le '
    my @vlans = (2, 3, 4, 7, 10, 11, 12, 20);
    my $last = $vlans[0] - 2;
    my @ranges;
    push @{$ranges[$#ranges + ($_ - $last > 1)]}, $last = $_ for @vlans;
    print join ",",
        map { $_->[0] == $_->[-1] ? $_->[0] : join "-", @{$_}[0, -1] } @ranges;
'
2-4,7,10-12,20