%hash = ( 1 => [ 51, 'a2'], 2 => [42, 'a1'] );;
# NOTE: ^______ parens not curlies _________^

print "$_ => @{$hash{$_}}" 
    for sort{ $hash{$a}[0] <=> $hash{$b}[0] } keys %hash;;

2 => 42 a1
1 => 51 a2