Most likely, qx or backticks will do, or IPC::Run, if you want to bring out the heavy guns. Also, you could try simply redirecting the output of the C++ program like this:

my $outfile = 'outfile.txt';
system("cplusplusprogram >$outfile") == 0
    or die "Couldn't launch cplusplusprogram: $!/$?";

open my $fh, "<", $outfile
    or die "Couldn't read '$outfile': $!";
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