The return value is the exit status of the program as returned by the "wait" call. To get the actual exit value, shift right by eight.

...

Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason).

So if nmake returns a failure it should return a positive integer, not -1, which would only returned if nmake wasn't found or similar.

I would code this like that: (untested)

my $pid = open (NMAKE, '-|', "nmake .... 2>&1");
if (!defined $pid) {
   warn "Couldn't start nmake";
}
while (<NMAKE>) {
    print STDERR $_;
    print STDOUT $_;
}
close (NMAKE);
my $status = $?;
if ($status) {
   # ...
}
[download]