You can't print user input to either the mail message or in the response HTML page? Are you sure you're getting the information, then? Try printing $query->Dump somewhere and see if the CGI object has in it what you expect.

As for error checking, using or die "error..." should do the trick:

my $query = new CGI
  or die "Couldn't create CGI object!\n";
open(MAIL, "|/usr/sbin/sendmail -t")
  or die "Couldn't start sendmail: $!\n";
close(MAIL)
  or die "Error closing sendmail: $!, exit status $?\n";
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