open (FILE, ">data.txt") or die "Oops! Perl whines: $!\n";
for my $i (0..$#screenres) {
     rename "$base_dir/$screenres[$i]", "$base_dir/".$i."_sc.jpg" or die "Rename failure, Perl moans: $!\n";
     print FILE "Renamed $base_dir/$screenres[$i] to $base_dir/".$i."_sc.jpg\n";
     # continue in the same vein as required
}
close FILE;

##</code><code>##

# opens data.txt for writing, *erases* old data.txt
open (FILE, ">data.txt")

# opens data.txt for appending, adds data to end of data.txt 
open (FILE, ">>data.txt")