Whole file copying i suggest using File::Copy. Basicly you can do this.

use File::Copy;

$A_file = $q->cookie('cookie_1');
$B_file = "file.txt";
copy ($B_file,$A_file);
[download]

Alternative way (probably slower though).

open FILE, "$B_file" or die "Unable to open \'$B_file\': $!";
open(OUT,">$A_file") or die "Unable to open \'$A_file\':
while (<FILE>) {
  print OUT "$_";
}
[download]

Btw, if you want to remove old, then use rename command (instead of examples above).