undef will stringify as "" so they can be used as hash keys, equivalent to an empty string.

warnings will complain though: 

perl -we'$b=undef;$a{$b}=1;print$a{""}'
Use of uninitialized value in hash element at -e line 1.
1

[download]

update: strictly speaking a hash key (for a normal hash) can't be undef, a reference, an object or anything but a plain string, and using anything as a hash key that isn't a string will use the stringified form. Which explains the behaviour in TedYoung's example.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: undef as a key in a hash. by Joost
in thread undef as a key in a hash. by fenLisesi

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