first of all: remember to use start_multipart_form and not just start_form() when you generate the input form, as in

#!/usr/bin/perl -w

use CGI qw/:standard/;

print header();
print start_html();
print start_multipart_form(-action=>'test.pl');
print filefield('file_name'), submit();

print end_form;
print end_html;
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then (test.pl)

#!/usr/bin/perl -w

use strict; ## ALLWAYS!!!!
use CGI;
use CGI::Carp qw(fatalsToBrowser);
$CGI::POST_MAX=1024 * 25;
my $q = new CGI;
my $file_name = $q->param('file_name');
my $fh = $q->upload('file_name'); # this is needed because use strict
if($fh) {
   open OUT, ">/tmp/output.gif" || die "cannot open: $!";
   binmode OUT; #just in case you are under windows
   print $q->header('image/gif');
   my $data;
   while(read($fh,$data,1024)) {
     print $data;
     print OUT $data;
   }
   close OUT;
}
else {
  print $q->header();
  print $q->start_html();
  print $q->h1('no upload file');
  print $q->end_html();
}
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