Is your collection an array-like (not keyed) or hash-like (keyed)? If your collection is hash-like, this would be trivial.

my @must_haves = qw( apple orange pear );

foreach my $collection (@collections) {
   foreach my $must_have (@must_haves) {
      $collection->{$must_have} = ...
         if not exists $collection->{$must_have};
   }
}
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If your collection is array-like, it's simplest to create a hash from it. See jhourcle's reply for an implementation.