while (<file1>){
   $line1=$_
   while(<file2>){
      $line2=$_
      foreach $field1 in $line1
         foreach $field2 in $line2
            if $field1 eq $field2{
               print $field1
            }
         }
      }
   }
}

##</code><code>##

while (<file1> or <file2>){ #The longest of the two files
   $line1=read ($file1);
   $line2=read ($file2);
   foreach $field1 in $line1
      foreach $field2 in $line2
         if $field1 eq $field2{
            print $field1
         }
      }
   }
}