perceptualdiff: 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/perceptualdiff - from package: perceptualdiff

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

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