Try:

    my $rv = $read_inputs->Call ($buf);
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Oh, and note that ReadInputs returns a list of values so print will print them concatenated together without any white space. You could:

my @values = ReadInputs();

print "@values\n";
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to at least get a space between each value. Note that the first value is the 'bool' result of the call and that the following values are the ADC input values.