Since you're renaming in the same directory, using File::Copy is overkill. The use of system is inefficient when Perl has a rename function built in, and using readdir for this purpose is probably unnecessary.

rename($_, lc($_)) foreach <*.HTM{,L}>;   # yes, this is portable
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I think it was Larry Wall that pushed a real simple command-line regex-based "rename" utility like this:

$op = shift or die "Usage: rename expr [files]\n";
chomp(@ARGV = <STDIN>) unless @ARGV;

for (@ARGV) {
   $was = $_;
   eval $op;
   die if $@;
   rename($was, $_) unless $was eq $_;
}
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Which would let you run something like:

rename '$_ = lc' *.HTM *.HTML
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