$text = "file1.txt";
$output = $text;
while (-f $output){
   $output .= ".new"
}
open(DAT,"$text") or die "Couldn't open $text for read: $!";
open(OUT,">$output") or die "Couldn't open $output for write: $!";
#your code goes here
#stuff like while(<DAT>)
#and print OUT $stuff_to_print;