You can do that by buffering the last "B" lines.

my $B  = 5;
my $A  = 3;
my $re = qr/somepat/;

my $skip = 0;
my @lines;
while (<>) {
   if (/$re/) {
      @lines = ();
      $skip = $A;
   }

   elsif ($skip) {
      --$skip;
   }

   else {
      push @lines, $_;
      print(shift(@lines)) if @lines > $B;
   }
}

print for @lines;
[download]

For test data, I used

keep 1
discard B5
discard B4
discard B3
discard B2
discard B1
somepat
discard A1
discard A2
discard A3
keep 2
keep 3
discard B5
discard B4
discard B3
discard B2
discard B1
somepat
discard A1
somepat
discard A1
discard A2
discard A3
keep 4
discard B5
discard B4
discard B3
discard B2
discard B1
somepat
discard A1
discard A2
discard A3
discard B1
somepat
discard A1
discard A2
discard A3
keep 5
[download]

For output, you should get

keep 1
keep 2
keep 3
keep 4
keep 5
[download]