@bigram = ( 'aa' .. 'zz' ); # magic!

##</code><code>##

# make a "set" of the ones in the list:
my %bigram_list;
@bigram_list{@bigram_list} = ();

# corresponds to (and same size as) @bigram.
my @bigrams_existing = 
  map { exists $bigram_list{$_} ? 1 : 0 }
  @bigram;