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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);

$VAR1 = [
          [
...
            'a2'
          ]
        ];