@foo{...} (note the curly braces) is Perl syntax for a "hash slice". The undef @unique{ @array } sets each key of %unique that is in @array to undef. This is a common and highly efficient way to create a list of keys in a hash, if you don't care about the value. As the goal is to make the list in @array unique, using the keys is the common approach as keys are always unique.

You can inspect what happens to %unique by using Data::Dumper:

use Data::Dumper;
print "Before: " . Dumper \%unique;
@unique{ @array }= undef;
print "After: " . Dumper \%unique;

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In reply to Re: What does undef do when I say @hashslice{@array} = undef by Corion
in thread What does undef do when I say @hashslice{@array} = undef by pritesh

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