There is more than one wayss to do it...

e.g.

sub ReadFile {
  my ($filename) = @_;
  local $/; # undef $/ for slurping in whole file

  open (FILE, $filename) or 
    die "Error in reading from $filename: $!\n";
  
  my $content = <FILE>;
  close (FILE);

  $content =~ s/^(?!--|\n).*\n//mg; # => see japhy's answer
  return ($content);
} # ReadFile
[download]

or reading into arraymode and then building scalar (might be faster, but haven't tested it).

sub ReadFile {
  my ($filename) = @_;

  open (FILE, $filename) or 
    die "Error in reading from $filename: $!\n";
  
  my $content = "";
  while (<FILE>){

    if ($_ eq "\n"    # if empty line
      or /^\-\-/){    # or -- at the beginning
      $content .= $_; # then append
    } # if

  } # while
  close (FILE);
  return ($content);
} # ReadFile
[download]

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"