#!/usr/bin/perl
BEGIN {
    $|=1;
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I don't see any compelling good reason to put all this in BEGIN.

    print "Content-type: text/html\n\n";

Don't! The whole point of CGI.pm is to relief you from the need of reinventing the wheel and the risks of doing it wrong.

    use CGI::Carp('fatalsToBrowser');
}

use CGI;
use LWP::Simple;
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Please (do a favour to yourself and) ask perl to give you all the help it can! So

use strict;
use warnings;
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as well.

$q= new CGI;
$my_data=$q->param('set');
$lower=1000; 
$upper=2000000; 
$random = int(rand( $upper-$lower+1 ) ) + $lower; 
$filename="file$random.jpg";
getstore($my_data,"../temp/$filename");
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Huh?!? Try reading

perldoc -q temporary

it may help.

However chances are you're having an OS problem, not a Perl one. You say that the program doesn't work, but fail to explicitly mention what kind of error it gives you. I suggest you to do your best to help knowledgeable monks here to help you...