Made the perl bindings ReadProperty example more generic

bacnet-stack/demo/perl/example_readprop.pl

@@ -1,16 +1,53 @@
 use warnings;
 use strict;
 
-if (scalar(@ARGV) == 1)
-{
-    my $device = $ARGV[0];
-    my ($resp, $failed) = ReadProperty($device, 'OBJECT_ANALOG_VALUE', 0, 'PROP_PRESENT_VALUE');
+my (
+    $device,     # device instance number
+    $objectName, # object type name
+    $objectInst, # object instance number
+    $propName,   # property name
+    $index,      # property index
+);
 
-    print "status was '$failed' and the response was '$resp'\n";
-}
-else
-{
-    print "Must specify device instance number as an argument to this script\n";
+GetOptions(
+    'device=i'   => \$device,
+    'objName=s'  => \$objectName,
+    'objInst=i'  => \$objectInst,
+    'property=s' => \$propName,
+    'index=i'    => \$index,
+);
+
+Help() unless ( defined($device)     &&
+                defined($objectName) &&
+                defined($objectInst) &&
+                defined($propName)
+);
+
+my ($resp, $failed) = ReadProperty($device, $objectName, $objectInst, $propName, $index);
+print "status was '$failed' and the response was '$resp'\n";
+
+sub Help {
+    print <<END;
+
+This script demonstrates the ReadProperty service functionality using Perl
+bindings. To run this script, you must specify the following arguments to it:
+  * device   This is the device instance number (i.e. 1234)
+  * objName  This is the object type name (i.e. OBJECT_ANALOG_VALUE). See
+             include/bacenum.h for complete list
+  * objInst  This is the object instance number you want to read (i.e. 1)
+  * property This is the name of the property you want to read (i.e.
+             PROP_PRESENT_VALUE). See include/bacenum.h for complete list
+  * index    This is an optional parameter. If you want to read from a specific
+             index, then specify here (i.e. 1). Otherwise, don't specify this
+             option.
+
+  As a complete example, to run this script using the main bacnet tool to read
+  AnalogValue1.PresentValue from device instance 1234, use
+
+perl bacnet.pl --script example_readprop.pl -- --device=1234 --objName=OBJECT_ANALOG_VALUE --objInst=1 --property=PROP_PRESENT_VALUE
+
+END
+    exit 1;
 }
 
 1;