my $start = qr/A{6}/;
my $end   = qr/B{6}/;
while ( <DATA> ) {
    if ( m{$start} .. m{$end} ) {
        next if m{$start};
        next if m{$end};
        print;
    }
}
__DATA__
hello world0
hello world1
hello world2
AAAAAA
this is a test of the
emergency broadcast system
BBBBBB
hello world0
hello world1
hello world2
AAAAAA
if this had been a real
emergency more instructions
would have followed
BBBBBB
hello world0
hello world1
hello world2
__END__
this is a test of the
emergency broadcast system
if this had been a real
emergency more instructions
would have followed