use Tie::Hash::Sorted;

my $custom_sort = sub {
    my $hash = shift;
    [ sort { $hash->{$b}{pkts} <=> $hash->{$a}{pkts}
                                ||
      $hash->{$b}{size} <=> $hash->{$a}{size} } keys %$hash
    ];
};
tie my %flows, 'Tie::Hash::Sorted', 
    'Sort_Routine' => $custom_sort,
    'Optimization' => 'none'
;
for ( keys %flows ) {
    # Will always be sorted the way you want
}