The important thing here is that variables used as keys are stringified. So, objects will be stringified. Also, undef will be stringified, as demonstrated: 
my %x;
my $u = undef;
my $s = "";

$x{$u} = 1;
$x{$s} = 2;

print "$x{$u}\n";

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These aren't limitations on what can be used, but consequences of using certain values.
My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

In reply to Re: Hash key string restrictions? by dragonchild
in thread Hash key string restrictions? by Ojosh!ro

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