use PDL;
@observed = ("ab", "ab", "ad", "an", "bd", "bn", "dn");
$ngramOrder = 2;
$alph = "{a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z}";
@perms =  glob $alph x $ngramOrder;

foreach $obs (@observed) { 
   push @hits, grep { $perms[$_] eq $obs } 0..$#perms;
}

@permCounts=list(zeroes($#perms+1));

foreach $hit (@hits) { 
   $permCounts[$hit]++;
}