So you're running the program in CGI context? What does the error_log of the web server tell you?

Some hints about your code:

• use strict and warnings; they support you with your development
• use the CGI module to obtain the given parameters.
• print a valid html header and a short message to STDOUT at the end of the script to prevent "internal server error", because no data was sent.

untested example

#! /usr/bin/perl
use strict;
use warnings;
use CGI;

my $lost_file = "/FULL/path/to/lost.txt";
my $cgi = CGI->new;

open LOST, '>>', $lostfile or die "open(>>) $lostfile failed: $!\n";

for my $param ( $cgi->param ) {
  print LOST "$param ", $cgi->param($param), "+";
}
print LOST "\n";

print $cgi->header('text/plain'),
  "job is done.\n";
[download]