use strict;
use File::Copy;
use constant REPLACEMENT_FILE => 'replacer_file.html';
use constant DIRECTORY => 'C:/foo/';

opendir(FOO, DIRECTORY);
my @file_list = grep /\.html?$/, readdir FOO;
for my $file (@file_list) {
  unless ($file eq REPLACEMENT_FILE) {
    unlink(DIRECTORY . $file);  #remove original
    copy DIRECTORY . REPLACEMENT_FILE, DIRECTORY . $file; #make copy of replacer & rename to original
  }
}
closedir FOO;