To put it simply, I think that what you want is one of:

# run xx.exe word_up and get the output of that program in a variable
my $xx_output = `xx.exe word_up`;

# just plain run the program... let the output go to the terminal
# just as it would be if you ran it at your dos prompt
system "xx.exe word_up";
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You're sort of confusing the program's input stream from it's command line. (In perl, this is the difference between STDIN and @ARGV... in C you'd call it stdin and argv/argc... whatever). It's the difference between (again at a command prompt) doing:

echo stuff | foo.exe
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(where "stuff" is going to be the input to foo.exe) and doing:

foo.exe stuff
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(where "stuff" is the first parameter to foo.exe). And it's also possible that you're additionally confusing it with:

foo.exe < stuff
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(where "stuff" is the name of a file, the contents of which are to be the input to foo.exe).