This is the reverse of a question in perlfaq4, namely, How can I tell whether a list or array contains a certain element?.

For example, you might say:

my %unmatches = ( 1 => 1, 2 => 1, 3 => 1, 4 => 1 );
unless (exists $unmatches{$sum} ) {
    print "didn't match!\n";
}
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