use a hash to filter doubles

#! perl -w
use strict;

my($file);

my(%upload,%seen);

%upload=(a=> 'dog',
        b=> 'cat',
        c=> 'llama',
        d => 'dog'
        );




for (keys %upload){
        
        $file=$upload{$_};
        
        if(defined $seen{$file}) {
            print "<center><font color=red>Duplicate filename found: $
+file</font></center>\n";
            exit;
          }
          
      $seen{$file}=$file;
      print "<font color=blue><center>Uploaded $file</font></center>";
+ 
          
      
  }
[download]

Instead of the exit call you could just make the filename more unique by tagging on a number (much like your OS does when you make try to make a file with a non unique name)

#! perl -w
use strict;

my(%upload,%seen);
my($counter,$file,$old);



%upload=(a=> 'dog.gif',
        b=> 'cat.gif',
        c=> 'llama.gif',
        d => 'dog.gif'
        );




for (keys %upload){
        
        $file=$upload{$_};
        
        if(defined $seen{$file}) {
            $old=$file;
            while (defined $seen{$file}){
                $file=$old;
                $counter++;
                $file=join('.',$file,$counter);
            
            }
            
          }
          
      $seen{$file}=$file;
      print "<font color=blue><center>Uploaded ",$file,"</font></cente
+r>\n"; 
          
      
  }
[download]