NetWallah showed how to recreate the hash, and GrandFather demonstrated "...how to extract data from the hash...:"

print "At $hash{alerts}[0]{timestamp}: $hash{alerts}[0]{text}\n";
                   ^    ^      ^
                   |    |      |
                   |    |      + - hash key
                   |    + - zeroth element of array
                   + - hash key

# {} means hash; [] means array
[download]

If you add and run the following line below the initialized hash:

say "$_ => $hash{$_}" for keys %hash;
[download]

it'll generate the following output:

alerts => ARRAY(0x2562880)
recovery_alerts => 0
bad_alerts => 1
[download]

The key recovery_alerts is associated with the value 0, and the key bad_alerts is associated with the value 1. However, the key alerts is associated with an array reference, thus the 'alerts' => [... notation above in the hash initialization.

So, what's in that array--or at least the zeroth element? Try the following (the [0] notation dereferences the array reference, specifying element 0):

print $hash{alerts}[0];
[download]

Output:

HASH(0x379350)
[download]

It's a hash reference, and the hash begins here in the hash initialization:

'alerts' => [
                {
                ^
                |
                + - beginning of hash
[download]

We can print the keys of this hash with the following:

say "$_ => $hash{alerts}[0]{$_}" for keys %{$hash{alerts}[0]};
[download]

Output:

min_failure_count => 3
status_history => HASH(0x4ec628)
status_code => 1
original_status_code => BAD
dimensions => HASH(0x4f9440)
best_group => NO GROUP
timestamp => 1347395226
text => 95.019 (value) > 95 (max limit) between Tue 20:19 - Tue 20:
+20 (UTC)
window_size => 3
check_type => system.fs-used_pct
[download]

Remember that $hash{alerts}[0] contains a hash reference. We enclose this with %{} to dereference it to get its keys in the line above. Each key (the values taken by $_) from the hash is 'plugged' into $hash{alerts}[0]{$_}, to get the key's associated value.

Does the structure $hash{alerts}[0]{$_} look familiar? Look back to the line GrandFather created:

print "At $hash{alerts}[0]{timestamp}: $hash{alerts}[0]{text}\n";
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^
[download]

From the structure you provided, he showed "...how to extract data from the hash..."

Hope this helps!