@bigram = ( 'aa' .. 'zz' );
@bigram_list = ("ac" , "ab" , "wd" , "xs" ); 
$bigram_str = join " " @bigram;
my %bigram_exist;

foreach $bigram_test (@bigram_list)
      { 
      if ( $bigram_str =~ m/$bigram_test/
           {
           $bigram_exist{$bigram_test}=1;
           }
      else 
           {
           $bigram_exist{$bigram_test} = 0;
           {
      }