As davido and Fletch have pointed out above, you should really be using Mail::Mailer.

But as to the error message: those are coming not from Perl, but from the shell that perl is spawning to run sendmail. You're using unquoted angle brackets in the email addresses, which the shell interprets as redirection symbols. Except there's no file name or symbol after the >, just a newline, which is a ... wait for it ... syntax error because the newline is not expected.

You could fix it by escaping the angle brackets, but as the others have mentioned, it'd probably be better to go with Mail::Mailer.

Update
Cleaned up text spacing a little.