Since @userArray2 is empty, it doesn't add anything to @mainArray

my @userArray = qw(batman robin joker);
my @userArray2; #this is a null array

my @mainArray = (@userArray, @userArray2);
print("Number of elements: ", scalar(@mainArray), "\n");   # 3
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If you want to filter out elements of an array, grep is usually the way to go.

my @mainArray = ('batman', 'robin', undef, 'joker');
print("Number of elements: ", scalar(@mainArray), "\n");   # 4
@mainArray = grep { defined } @mainArray;
print("Number of elements: ", scalar(@mainArray), "\n");   # 3
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By the way, adding Array to your variable names is redundant. That's what @ means.