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$str1='abcde';
$str2='eda';
...
$diff1=$str1=~tr/$str2//c;
$diff2=$str1=~tr/eda//c;
print "   diff1=$diff1, diff2=$diff2\n";

Test 1: strait set
   diff1=0, diff2=3
Test 2: complement set
   diff1=5, diff2=2