factor.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/bin/factor.pl - from package: libmath-prime-util-perl

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

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