use File::Copy;

opendir(DIR,$file);
my @files = readdir(DIR);
close(DIR);

for my $file (@files) {
    if($file =~ /\.HTML?$/) {
        my $new_file = lc $file;
        move($file, $new_file) or
           die "Can't move file: $!";
    }
}
[download]

Update: Fixed incorrect Perl construct. Thanks to gav^ for noticing it.

--
Ilya Martynov (http://martynov.org/)

Here is something that would work

opendir(DIR, $dir) or die "Cannot open directory '$dir': $!";
my @files = readdir(DIR);
close(DIR);

foreach my $file (@files) {
    if ($file =~ /\.HTML?$/i) {
        system("rename $file \L$file\E");
    }
}
[download]

gav^

A couple of things here:

1. $file is not the current item in the @files array.
2. The function call will not work inside the quotes.

This is untested, but I'd go with something more like this:

my $file = '.' ;

opendir( DIR, $file ) ;
@files = readdir( DIR ) ;
close( DIR ) ;

for ( @files )
{
    if ( /\.HTM[L]?/ )
    {
        system("rename", $_, lc( $_ ) );
    }
}
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Or, even better (if it's supported on NT), just use Perl's rename command.

rename $_, lc( $_ ) ;
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rename($_, lc($_)) foreach <*.HTM{,L}>;   # yes, this is portable
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$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 $_;
}
[download]

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