installer.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/share/otrs/bin/cgi-bin/installer.pl - from package: otrs2
/usr/share/otrs/bin/fcgi-bin/installer.pl - from package: otrs2

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

Error in other languages:
installer.pl: bevel nie gevind nie
installer.pl: comandă negăsită
installer.pl: comando não encontrado
installer.pl: commande introuvable
installer.pl: command not found
installer.pl: käsku ei ole
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installer.pl: komanda nerasta
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installer.pl: kommandot finns inte
installer.pl: komut yok
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installer.pl: no se encontró la orden
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installer.pl: opdracht niet gevonden
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installer.pl: perintah tidak ditemukan
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installer.pl: příkaz nenalezen
installer.pl: команда не найдена
installer.pl: командата не е открита
installer.pl: 命令找不到
installer.pl: コマンドが見つかりません