Well, you can only get the exe's return value if you are expecting a single integer back from the call.

my $returncode = system('bar.exe some params');
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I think there are some rather harsh range limitations on the return code as well. That's probably not going to be good enough, so you're probably going to have to develop a common interface of some kind. The easiest would be to simply print to stdout, but it could be anything, like creating a file, or inserting into a database, or whatever.

my $stdout = qx("bar.exe some arguments");
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Watch out for the fact that command-line arguments are processed differently depending on how many arguments you pass to the system or qx call.