app.psgi: command not found

Have You tried accessing the binary with the full path?

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/doc/libmagpie-perl/examples/debug_xslt/bin/app.psgi - from package: libmagpie-perl
/usr/share/doc/libpoet-perl/examples/blog/bin/app.psgi - from package: libpoet-perl
/usr/share/perl5/auto/share/dist/Poet/generate.skel/bin/app.psgi - from package: libpoet-perl
/usr/share/otrs/bin/cgi-bin/app.psgi - from package: otrs2

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