Thanks to all you monks for the precious suggestions you gave to me!
I know my code is still terrible!!

#!perl

use strict;
use warnings;

&changemyself();
sub changemyself{
  my $new_value = 10;
  my $data=6666666666666;   #next time this value will be 10

 open (COPIA, ">copia");
 open (PROG, "+>>$0");
 seek (PROG, 0, 0);         #I don't know why but it's NECESSARY!

 while (<PROG>)
 {
   s/^(  my .data\s*=\s*)\d+(\s*;)/$1.$new_value.$2/e;
   print COPIA;
 }
 close (PROG);              #
 close (COPIA);             #
                            #   the terrible imitation 
                            #     of the
 open (COPIA, "<copia");    #   three cards game
 open (PROG, ">$0");        #
 while (<COPIA>){print PROG}#
 close (PROG);              #
 close (COPIA);             #
 unlink ("copia");          #
}
[download]

Special thank to Helter!
greetings from roma, lor*
PS I never noticed the +>> operator!!! l*