my %hash = (foo => [1..2], bar => [1..3], baz => [1]);
my @keys = sort { @{$hash{$a}} <=> @{$hash{$b}} } keys %hash;
print "$_ - @{$hash{$_}}", $/ for @keys;

__output__

baz - 1
foo - 1 2
bar - 1 2 3