The brackets are being intercepting by the shell. You have to properly quote shell metacharacters... actually, as there are two shells being invoked (the one used by perl to run your command and the one you are calling explicitly), you will have to double quote your arguments.
in reply to Re^5: Unable to connect to some SFTP servers using Perl::SFTP
in thread Unable to connect to some SFTP servers using Perl::SFTP
If you want to follow that route and need help about quoting I advice you to post a new question.