• Uses flock to handle multiple users.
• Added use strict;, which revealed many errors.
• Stopped using globals for every variable.
• Localized changes to required globals.
• Fixed bug where $filename and $filedata weren't always defined.
• Used safer 3-arg open.
• Used or instead of || when used instead of if.

use strict;
use warnings;

use Fcntl ':flock';  # Import LOCK_* constants

sub getFile {
   my $fileName = shift(@_);

   unless (defined $gotFiles->{$fileName}) {
      local $/;
      open(my $fh, '<', $fileName)
         or die;
      flock($fh, LOCK_SH)
         or die;
      $gotFiles->{$fileName} = <$fh>;
      # File handle automatically closed and unlocked here.
   }

   return $gotFiles->{$fileName};
}

sub writeFile {
   my $data = shift(@_);

   my ($filename) = $data =~ m@<filename>(.*?)</filename>@s
      or return 0;
   my ($filedata) = $data =~ m@<data>(.*)</data>@s
      or return 0;

   {
      open(my $fh, '>', "$ENV{DOCUMENT_ROOT}/$filename")
         or return 0;
      flock($fh, LOCK_EX)
         or return 0;
      print $fh $filedata; 
      # File handle automatically closed and unlocked here.
   }

   $gotFiles->{"$ENV{DOCUMENT_ROOT}/$filename"} = $filedata;
   return 1;
}
[download]