Do you want to change just one specific file? If that is so just use the methods already explained to you. However, if you want to look through a directory and then change any files that don't have an extension on them you could try something like this:

#!perl
use strict;

my @file = glob "*";
foreach(@file){
  if(/\./){next;}
  else{
    rename $_ , $_ . ".rtf";
    }
}
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If you need to change the files in different directories you could simply recieve a command line argument that would contain the directory you need to evaluate and the do a chdir $ARGV[0], or the equivalent.