• Try using the upload method if it's available in your version of CGI; that's the recommended interface nowadays.
• Add some error checking to see if a file is being uploaded, and if that is detected correctly. That may well get you to the source of the problem, and even if not it will make your code more robust.
• The fileno function will tell you the file descriptor of a file handle, and might be helpful in debugging.

Good luck!

#!/usr/bin/perl
use warnings;
use strict;
use CGI::Simple;

my $maxsize = 1024 * 100; #max 100K 
my $upload_dir='uploads';
my $q = new CGI::Simple;
print $q->header();

if($ENV{CONTENT_LENGTH} > $maxsize){
 print "file too large - must be less than $maxsize bytes";
 exit;
 }

my $files    = $q->upload();                # number of files uploaded
+; 
my @files    = $q->upload();               # names of all uploaded fil
+es 
my $filename = $q->param('upload_file');    # filename of uploaded fil
+e 
my $mime     = $q->upload_info($filename,'mime'); # MIME type of uploa
+ded file 
my $size     = $q->upload_info($filename,'size'); # size of uploaded f
+ile 

# short and sweet upload 
my $ok = $q->upload( $q->param('upload_file'),"$upload_dir/$filename")
+;
print "Uploaded ".$q->param('upload_file')." and wrote it OK!\n" if $o
+k;

print "total files = $files<br> 
       filenames = @files<br> 
       filename = $filename<br> 
       mimetype= $mime<br> 
       size=$size<br>";
[download]

In the first instance you probably want to see the section entitled CREATING A FILE UPLOAD FIELD in the CGI documentation.

/J\