#! /usr/bin/perl

use strict;
use warnings;

my $A = 2;
my $file = '15ug.txt';
open(my $fh,"<$file") or die $!;
while(<$fh>){
   print if(/5ug$A/.../6\*End/);
}
close $fh;

5ug1
6    1010 aaaaaaa =    "1234" bbbbb
6    2020 bbbbbb =  "9876" cccccc
6*End
5ug2
6    1020 aaaaaa = "1234" bbbbbb
6    2030 bbbbbb = "9876" cccccc
6*End
5ug3
6    1010 aaaaaaa =    "1234" bbbbb
6    2020 bbbbbb =  "9876" cccccc
6*End
5ug4
6    1020 aaaaaa = "1234" bbbbbb
6    2030 bbbbbb = "9876" cccccc
6*End

5ug2
6    1020 aaaaaa = "1234" bbbbbb
6    2030 bbbbbb = "9876" cccccc
6*End