Why not just send the code straight to "perl -cw" itself?

sub test_code {
   my $code_to_check = shift;

   my $pid = open(PERL, "|-");
   if (!$pid) {
      die "fork: $!" unless defined $pid;
      exec("/usr/bin/perl", "-cw") or die "/usr/bin/perl: $!";
   }

   print PERL $code_to_check;
   close(PERL);

   return !$?;     # $? == 0 means compile succeeded
}
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You may want to close STDIN/STDERR to handle any output the forked Perl process gives, and just rely on the exit status, or use open2/open3 if you want to capture the compilation messages Perl gives you.