This works for me

use strict;

use warnings;

use Data::Dumper;

foreach my $line (<DATA>) { 
    chomp($line);
    $line =~ s/^A$|^B$//;
    if ( $line ) { print "$line\n"; }
}

__DATA__
A
ZZZ
YYY
XXX
B
A
B
A
FFF
SSS
GGG
B
A
B
A
WWW
EEE
TTT
BCC
[download]