Why doesn't Perl's "sort" put these hash keys in numeric order?

Question

Consider:

#!/usr/bin/perl
use strict;
use warnings;

my %hash;
foreach (1 .. 10) {
    $hash{$_} = $_;
}
foreach(sort(keys %hash)) {
    print $_ . ":  " . "$hash{$_}" . "\n";
}

When I execute the above code, the result is as below:

1:  1
10:  10
2:  2
3:  3
4:  4
5:  5
6:  6
7:  7
8:  8
9:  9

I expect "10: 10" to be the last one that is printed. Why does Perl give me a surprise in this case?

Solution

sort always defaults to string comparison.

If you want a numeric sort, you have to be explicit.

sort {$a <=> $b} (keys %hash)

Comments

[jrockway]

A hash with keys 1..10 is called an array.

[Shailesh]

Perl stores them both as strings and as numbers. The problem here is that by default sort sorts the strings.

[Jacob]

The keys of a hash are stored as strings, but even if they were stored as numbers sort still sorts using string comparison.

[brian d foy]

Hash keys aren't SVs, so they don't have the same magic as normal Perl scalars. This is why, for instance, you can untaint strings by using them as a hash key and getting them back out with keys().

[andeyatz]

If my current understanding is correct aren't they stored as const char* key or at least the C methods for interacting with hashes seem to want this.

[Thawn]

This answer would be way more helpful, if it provided a solution. Otherwise, just delete it, it does not add any useful information and leaves the reader to resort to Quentins answer for a solution and a link to the proper explanation.