# simplistic
my @lines = scalar <FILE>;
while (<FILE>) {
  if (s/^\+//) { $lines[-1] .= $_ }
  else { push @lines, $_ }
}


# fancy
my @lines;
while (<FILE>) {
  if (s/^\+//) { $lines[@lines && -1] .= $_ }
  else { push @lines, $_ }
}


# compact (UPDATED:  1 -> -1)
my @lines;
$lines[s/^\+// ? @lines && -1 : @lines] .= $_ while <FILE>;