Some idea of the complexity may be understood by saying how many distinct elements @a_list produces:

my $ref = [];
@a_list = ([[], []], [[], []], [$ref, $ref]);
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The easiest and therefore usual answer is, "They are the same if they yield the same strings in string context." (That would give 3 above.) That is reasonable. And it can be done with 21 characters:

sub r{
#23456789_123456789_1
my%e;grep!$e{$_}++,@_
}
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This one isn't mine (it came from Effective Perl Programming):

sub r{
  keys%{{map{$_,1}@_}}
}
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Nope -- it stringifies everything, which I understand to be contrary to specifications. However, it is inspiring:

sub r{
  values%{{map{$_,$_}@_}}
}
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This preserves references as references -- does that meet the specs?

The Sidhekin

But wouldn't two items collide in this hash if their stringified representations were the same? For instance, wouldn't an object that stringified to "abc" and the actual string "abc" refuse to coexist in that hash.

I don't see how using values gets around the stringified nature of the keys.

-Blake