Here one try I hope you can take it as a starting point.

I make the follwing assumptions:

- it is "##closeFlag## (without blank)

- your input is stored in a file called "in.txt"

- the input file is small enough to fit into memory

use strict;

# first slurp the file
my $content = do {
                    local $/;
                    open my $fh, "<", "in.txt" or die $!;
                    <$fh>
                 };

# then split it into chunks
my @chunks = $content =~ /##Flag##(.*?)##CloseFlag##/sg;

# now analyze the chunks
for my $chunk (@chunks) {
   my ($first, $last) = $chunk =~ /T:(\d+).*?(\d+)\D*$/s;

   # count number of newlines
   my $newlines = $chunk =~ tr/\n/\n/;

   print "first-last: ", $first - $last, "# of lines: ", $newlines-1, 
+"\n";
}
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