Whenever you think "exists", think of exists - which means using a hash. Put the existing files as keys in a hash (the value of the entry is irrelevant). Then grep against the should-exist list.

my %have_file;
undef @have_file{glob("*")};
my @dont_exist = grep !exists $have_file{$_}, @should_exist;
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On the second line, what I'm using is called a slice. Rather than selecting a single entry from the hash, I select a list - here, the list of filenames returned by glob (of course any other way you get your file names is applicable as well). Then I undef all of these hash entries into existence.

Actually the way I'd write this in my own code is like so:

my @dont_exist = do {
    my %have_file;
    undef @have_file{glob("*")};
    grep !exists $have_file{$_}, @should_exist;
};
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Makeshifts last the longest.