You are right. I ended up reading in the header file, line by line and splitting on the first colon ':'. Then, I use the take the info left of the colon and pair it with the info to the right. I build a HTTP::Headers object, and assign each left/right pair to the object via:

if ( -f $file_hdr_raw ){
   open(RAW, "<$file_hdr_raw") or plog("Can NOT open <$SHA1_file_hdr_r
+aw> for reading\n");
   my(@lines) = <RAW>;
   close(RAW);
   while ( @lines ){
      my $line = shift(@lines);
      my ($key, $value) = split(/:/, $line, 2);
      $headers->header( $key => $value );
      }
}
                
# create response
    my $res = HTTP::Response->new($status_code, $status, $headers, $co
+ntent);

# send back (short-circuit normal content fetching)
    $self->proxy()->response($res);
[download]

At least I have tested the above, mixed with other code, and it works, well. I turned on the proxy use with Firefox and used a FF plugin to verify I was receiving the complete content of a past header saved off.

Thank you to everyone for their help.