There are a few things that I would change. First,

my $latin2 = find_encoding("iso-8859-2");
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I'd change that to

my $enc = find_encoding("iso-8859-2")->name;
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That's because I hate to type:). You could do similarly with

my $userAgent = LWP::USerAgent->new;
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I'd do

my $ua = LWP::UserAgent->new;
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Then, to debug the script, I'd use something like Debug::STDERR. I put it in the script.

#!/usr/bin/perl

use strict;
use warnings;
use Encode;
use Getopt::Long;
use HTML::Form;
use HTTP::Request::Common;
use LWP::UserAgent;
use URI::Escape::JavaScript;

BEGIN {
    $ENV{DEBUG} = 1;
}
use Debug::STDERR;

my $url;
my $message = '';
my $scripts = '';
my $username;
my $password;
my $file;
my $name;

my $login = 0;
my $response;

my $usage =
"usage: upload.pl url [--message] [--scripts] [--username --password] 
+[--file [--name]]";

GetOptions(
    'message|m=s'  => \$message,
    'scripts|s=s'  => \$scripts,
    'username|u=s' => \$username,
    'password|p=s' => \$password,
    'file|f=s'     => \$file,
    'name|a=s'     => \$name
) or die $usage;

if ( @ARGV != 1 ) { die $usage }
$url = $ARGV[0];

if ( $username && $password ) { $login = 1; }
elsif ( $username || $password ) {
    die 'Username and password must be supplied together.';
}
if ($file) {
    if ( !-e $file ) { die "File not found: $file" }
}
elsif ( !$message ) { die 'Either a file or a message must be supplied
+.' }

my $ua = LWP::UserAgent->new;
debug( my_debug => { ua => $ua } );
$ua->agent('PerlKamion');
$ua->cookie_jar( {} );

my $enc = find_encoding("iso-8859-2");

#login
if ($login) {
    print "Logging in...\n";
    $response = $ua->post(
        'http://lohere.net/_user/bejelentkezes.php?login=true',
        [ user => $username, password => $password ],
        Content_Type => 'form_data',
    );

    if ( $response->filename == "bejelentkezes.php" ) {
        $enc->decode( $response->content ) =~
          m|<font color="red">(.+?)</font>|;
        die "Unable to log in: $1";
    }
}

# authentication
print "Authenticating...\n";
$response = $ua->get($url);
$enc->decode( $response->content ) =~ m|unescape\('(.+?)'\)|;
my @forms = HTML::Form->parse( js_unescape($1), $url );

# upload
my %uploadForm = $forms[0]->form;
$uploadForm{'longcat'} = $scripts;
$uploadForm{'is'}      = $message;

if ($file) { $uploadForm{'looong'} = [ $file, $name ] }
print "Posting...\n";
$response = $ua->post( $url, \%uploadForm, Content_Type => 'form-data'
+ );
my $responseText = $enc->decode( $response->content );

print $responseText;
if ( index( $responseText, "Oldal friss\u00edt\u00e9se..." ) == -1 ) {
    $responseText =~ m|<td align="left">(.+?)</td>|;
    die "Upload error: $1";
}
print "Done.";
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Update: Fixed encoding error.