my $digit_width = 4;
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 what?
    }
}