my @out;
my $print = 1;
while (<>) {
  if (m/^start_of_block/) {
    $print = 0;
    pop @out
     if @out and $out[-1] eq $/; # removes previous empty line
  }

  push @out, $_ if $print;

  if (m/^};/) {
    $print = 1;
  }

  print shift @out if $print and 1 < @out;
}

# UPDATE: based on oldmanwillow reply:
# print @out;
print @out if $out[-1] =~ /^\s*$/ ;