mperfmon: 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/mperfmon - from package: mono-tools-gui

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

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