That error means something (such as an error message) was printed before the header was printed. In this case, an error is printed before compilation of the script is done, and the header is only outputed after the script is done compiling.

The content of eval BLOCK is compiled when the rest of the script is compiled, so the use in the eval BLOCK (and anything in the used file) will be executed before you send the header.

You could send the header sooner (using BEGIN), or you could use eval EXPR instead of eval BLOCK.

The following executes everything in the same order as if the error checking wasn't there (i.e. modules are loaded before CHECK is called), but dies gracefully if a module can't be loaded.

#!/usr/bin/perl

BEGIN {
   eval ("
      use LWP::UserAgent;
      use HTTP::Request::Common;
   ");

   if ($@) {
      print "500 Internal Server Error\n";
      print "Content-type: text/plain\n";
      print "\n";
      print $@

      die($@);  # For the log file.
   }
}

print "Content-type: text/html\n\n";

my $ua = LWP::UserAgent->new;
my %post = (...);
my $response = $ua->request(POST "http://www.example.com/", [ %post ])
+;
print $response->as_string;
[download]

By the way, it's $@, not @$.