Don't add AUTOLOAD generated methods in symbol table (#409) (#410)

When a method is resolved via AUTOLOAD (eg for objects defined in %FUNCS),
adding the method to the symbol table leads to skipping AUTOLOAD on subsequent
calls to the method. The method table however is global to the interpreter,
so even when a new SNMP::Info instance is created for a different device, all
methods created by a previous device are just "carried over" to the new one
if they are put into the symbol table. This breaks a method like "v_name",
which may resolve differently in different device classes, in the use case
where devices from different classes are polled one after another (even if
new SNMP::Info object instances are used every time).

lib/SNMP/Info.pm

@@ -4989,8 +4989,7 @@ capabilities to include dynamic methods generated at run time via AUTOLOAD.
 
 Calls parent can() first to see if method exists, if not validates that a
 method should be created then dispatches to the appropriate internal method
-for creation.  The newly created method is inserted into the symbol table
-returning to AUTOLOAD only for the initial method call.
+for creation.
 
 Returns undef if the method does not exist and can not be created.
 
@@ -5027,15 +5026,21 @@ sub can {
     # we’ll just create if/when they are called rather than pollute the
     # symbol table with entries that never get called.
 
+    # Between 2012-2020 we actually added the methods generated below to the
+    # symbol table, but they were global to the SNMP::Info class, while
+    # methods for different device classes may point to different SNMP
+    # objects. This made interacting with multiple device types from a single
+    # script somewhat unreliable.
+
     # Check for set_ ing.
     if ( $method =~ /^set_/ ) {
-        return *{$method} = _make_setter( $method, $oid, @_ );
+        return _make_setter( $method, $oid, @_ );
     }
     elsif ( defined $funcs->{$base_method} || $table ) {
-        return *{$method} = _load_attr( $method, $oid, @_ );
+        return _load_attr( $method, $oid, @_ );
     }
     else {
-        return *{$method} = _global( $method, $oid );
+        return _global( $method, $oid );
     }
 }
 
@@ -5044,9 +5049,7 @@ sub can {
 =head2 AUTOLOAD
 
 Each entry in either %FUNCS, %GLOBALS, or MIB Leaf node names present in
-loaded MIBs are used by AUTOLOAD() to create dynamic methods.  Generated
-methods are inserted into the symbol table so that subsequent calls can avoid
-AUTOLOAD() and dispatch directly.
+loaded MIBs are used by AUTOLOAD() to create dynamic methods.
 
 =over