use Tie::Hash::Sorted;

my %hash = (
    5189 => 63, 
    3213 => 9, 
    2357 => 3,
);

tie my %sorted_hash, 'Tie::Hash::Sorted',
    Hash         => \%ages,
    Sort_Routine => sub { $a <=> $b },
);


sub next_key {
    my ($h,$key) = @_;

    foreach my $hkey (keys %$h) {
        return $h->{$hkey} if $hkey >= $key;
    }
    return;
}

print next_key(\%hash,3000) . "\n";