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;