my @Files = grep { /\.txt$/ && -f "$Dir$dirSep$_" } ...
...
-f && /\.txt$/ && ($fileCounter+=1);
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You've made a big benchmarking error. You are comparing two very different, though seemingly equivalent, operations. In your first test, you first do a relatively inexpensive operation first which in most cases short-circuits the need to do the relatively expensive operation second (assuming there are relatively few '.txt' files).

In the second test, you always perform the expensive operation (a file stat, or whatever the Windows equivalent is), which makes this an unfair comparison. Make the comparison fair and I think you'll be surprised.

BTW, it would take more than the 4-6% improvement that you cite for me to reinvent this wheel anyway :-)