Using your code:
my $digit_width = 4; # This isnt necessarily true every time
my $min = 1;
my $max = 12;
my $prefix = "test_";
my $ext = ".txt";

while ( my $file = readdir DIR ) {
    next unless ( $file =~ /^ $prefix (\d{$digit_width}) $ext $/x );
    if ( $1 >= $min and $1 <= $max ) {
        # we have a match... now now we want to create the new file name
        # Whatever $SetName was so if $SetName = "Chapter \1" which means 
        # The new file name for the program to modify in the directory is 
        # to "Chapter 01.txt" for the 1st chapter file which wouldve been
        # Test_001.txt in the directory it searched in. I will then
        # have the code rename the file.
    }
}