cert-req.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 CentOS possible default full paths are:

/usr/bin/cert-req.pl - from package: perl-VOMS-Lite

You may install the required package with command yum install {package}

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