Your post as been already discussed at length, I'd just like to add that (maybe you didn't know and may be glad to know that) instead of

open(FH, "wrongs") or die $!;
open(FH2, ">wrongs2") or die $!;
while ($line = <FH>) {
   $line =~ s/wrongs/wrongs3/;
   print FH2 "$line";
}
close (FH);
close (FH2);
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you can use some cmd line swithces and arguments and shell redirection (this particular one works in any common shell that I know of, including command.com):

perl -lpe 's/wrongs/wrongs3/' wrongs >wrongs2
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or

perl -lpi -e 's/wrongs/wrongs3/' wrongs
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if you want in place editing. But under Windows AFAIK you will have to do (something like):

perl -lpe "s/wrongs/wrongs3/" wrongs >wrongs2
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and

perl -lpi.bak -e "s/wrongs/wrongs3/" wrongs
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respectively instead; i.e. use double quotes for quoting and cannot do in place editing withou backup.

Modified my Perl code to count the number of replacements as well as added benchmarking:

The code above can easily be adapted to show the count:

perl -lpi '$c++ if s/wrongs/wrongs3/; END{warn "count=$c\n"}' wrongs
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For the benchmark, it's not just that easy. That's what Benchmark.pm is for, and indeed it works by repeating the code to be tested a suitable number of times. But if the task takes long enough, then bash's time command will suffice: here's a test on a file that's 3494270 lines long:

$ time perl -lpe '$c++ if s/sex/cool/; END{warn "count=$c\n"}' gse1.lo
+g >/dev/null 
count=2840

real    0m19.062s
user    0m16.736s
sys     0m0.940s
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Unfortunately it's not available under command.com and cmd.exe, that I know.

Last, you may also want to count the number of substitutions when the /g global modifier is given. In that case

perl -lpi '$c+=s/wrongs/wrongs3/g; END{warn "count=$c\n"}' wrongs
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or else you may use the "highly experimental" </c>(?{ code })</c> regex feature:

perl -lpi 's/wrongs($c++)/wrongs3/g; END{warn "count=$c\n"}' wrongs
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but there's really no need for it here...