With eval statement works correctly. So it looks like $ is treated by tr as a regular symbol and no warning are issued.

$statement='$diff1=$str1'."=~tr/$str2//;";
eval($statement);
print "With eval: diff1=$diff1\n";
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that will produce:

With eval: diff1=3
</code> 
Comment on Re: Strange behaviour of tr function in case the set1 is supplied by a variable
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