In order to make the second case work like the first one, you need to have the back-tick command assign its value to a scalar, rather than to an array:

my $data =`lpstat -o -v -d -p -l -D`;.
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In other words, if your first example (reading from the file) actually works, then all you need to do is replace these lines:

system("lpstat -v -d -p -l -D >junk.txt");
open RI,"<junk.txt" || die "Can't open junk.txt for reading:$!\n";
my $data = do { local $/; <RI> };
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with the one back-tick line I showed above, and everything else should stay the same and still work.