customer.pl: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/share/otrs/bin/cgi-bin/customer.pl - from package: otrs2
/usr/share/otrs/bin/fcgi-bin/customer.pl - from package: otrs2

You may install the required package with command apt-get install {package}

Error in other languages:
customer.pl: bevel nie gevind nie
customer.pl: comandă negăsită
customer.pl: comando não encontrado
customer.pl: commande introuvable
customer.pl: command not found
customer.pl: käsku ei ole
customer.pl: không tìm thấy lệnh
customer.pl: komanda nerasta
customer.pl: Kommando nicht gefunden.
customer.pl: kommandot finns inte
customer.pl: komut yok
customer.pl: nie znaleziono polecenia
customer.pl: níor aimsíodh an t-ordú
customer.pl: no se encontró la orden
customer.pl: no s'ha trobat l'ordre
customer.pl: opdracht niet gevonden
customer.pl: parancs nem található
customer.pl: perintah tidak ditemukan
customer.pl: príkaz nenájdený
customer.pl: příkaz nenalezen
customer.pl: команда не найдена
customer.pl: командата не е открита
customer.pl: 命令找不到
customer.pl: コマンドが見つかりません