$string = "KHI0339B__P_H_Vita_Korangi_Ind_A";
my $string_1 = "HGW6120A__S_Popalzai_Cross_A";
$string_1 =~ s/__[a-zA-Z]{1,2}_/__/g;

print ":$string_1:\n";

Output :-
:HGW6120A__gPopalzai_Cross_A: