How about this?

#!/usr/bin/perl -w
use strict;

my $file_contents;
while( <DATA> ){
  next unless ( /^--/ || /^$/ );
  $file_contents .= $_;
}
print $file_contents;

__DATA__
--keep me
delete me

-- don't not throw this one out.
-- delete every line that does NOT begin with two dashes...
-- is the same as keep every line that begins with two dashes
[download]

--HTH

s!!password!;y?sordid?binger?;
y.paw.mrk.;;print chr 0x5b ;;;
print;print chr(0x5b+0x2);;;;;
[download]