use Sort::Key qw(nkeysort);
use Data::Dumper;
my %hash = ( 1 => [ 51, 'a2'], 2 => [42, 'a1'] );
my @res = nkeysort { $_->[0]} (values %hash);
print Dumper(\@res);

##</code><code>##

$VAR1 = [
          [
            42,
            'a1'
          ],
          [
            51,
            'a2'
          ]
        ];