Well, you read the output from the handle. And by output, I mean whatever the program writes to STDOUT. If the program "outputs" by driving the terminal and printing to the terminal, it becomes something different.

open my $fh, "cat |" or die;
while (<$fh>) {
    print "Got: $_";
}
close $fh or die;
__END__
foo
Got: foo
bar
Got: bar
baz
Got: baz
^D
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