use strict;
use warnings;

my @strings = qw{
   KHI0339B__P_H_Vita_Korangi_Ind_A
   HGW6120A__S_Popalzai_Cross_A
   };

my $rxPatt = qr{(?<=__)[A-Z]_};
foreach my $string ( @strings )
{
    print qq{Original: $string\n};
    $string =~ s{$rxPatt}{}
       while $string =~ $rxPatt;
    print qq{Modified: $string\n\n};
}