my @userArray = qw(batman robin joker);
my @userArray2; #this is a null array
my @mainArray = (@userArray, @userArray2);
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and then...

#@set should consist "batman robin joker", but not
#"batman robin joker NULL"
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@mainArray should already contain what you want.

my @set = map { exists($_)? $_ : delete($_) } @mainArray;
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I think you misunderstand exists and delete.

-David