Do not use & unless you know what you are doing.

Running your code under strict and warnings yields a message about using defined for arrays and invites to use just @a.

use strict;
use warnings;

sub test {
  undef;
}

my @a = test;
if (@a) {
  use Data::Dumper; print Dumper \@a;
}
__END__
$VAR1 = [
          undef
        ];
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I hope this answers your question.