I think your explanations are very misleading ...

my %hash;
my @array = (1..100);

# this will be ok
$hash{\@array} = 42;
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What happens here is that \@array gets stringified, so the key in the hash is a string (not a reference!) that looks something like "ARRAY(0x987f468)".

Such a construction is (unfortunately) not causing an error, but it almost never is what you want, so it is not ok. It usually is a programming error.

my %hash;
my @array = (1..100);

# this will be ok
$hash{\@array} = 42;

# but this will issue an error, because \@array 
# cannot be used as a key which varies
$hash{\@array}{1} = 42;
[download]

What happens here is that you stringify \@array twice (resulting in the same string each time), so in $hash{\@array}{1} you try to use $hash{\@array} as a hash, but you have assigned 42 (which is not a hash) to that before. The error has nothing to do at all with \@array being used as a hash-key.

Witness:

use strict;

my %hash;
my @array = (1..100);
$hash{\@array} = {};
$hash{\@array}{1} = 42;
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This does not produce an error (but I would not say it is ok just because of that).

If you want to use references as hash-keys you have to use Tie::RefHash.