Didn't benchmark - but I believe tr/X/X/ is still modifying the string on the backend (unless there is some unknown optimization).

It is better to leave off the second X as in tr/X//. This will not attempt to modify the string and will still return the count - as in: 
perl -e '$a="aaa"; $i=$a=~tr/a//; print "($i)($a)\n";'

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my @a=qw(random brilliant braindead); print $a[rand(@a)];

In reply to Re^2: number of matches by Rhandom
in thread number of matches by Anonymous Monk

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