OK, I'm at another computer, same system as yours, no older files but copied your code and my replacement from this thread, and downloaded portable Strawberry Perl v5.16.3

C:\>perl rec.pl

Enter --> a=2 print 2
2

Enter -->

C:\>perl -MParse::RecDescent -wE "say $Parse::RecDescent::VERSION"
1.967015

C:\>perl -v

This is perl 5, version 16, subversion 3 (v5.16.3) built for MSWin32-x
+64-multi-thread
[download]

Edit: I think failure to parse should prevent any changes to your "symbol table" i.e. %VARIABLE and suppress output from "print":

# ...
   expression : INTEGER OP expression
              { main::expression(@item) }
              | VARIABLE OP expression
              { main::expression(@item) }
              | INTEGER
              | VARIABLE
              <defer: $main::VARIABLE{$item{VARIABLE}} ||= 0 >
              { $main::VARIABLE{$item{VARIABLE}}}
              
   print_instruction  : /print/i expression
                      <defer: print $item{expression}."\n" >
   assign_instruction : VARIABLE "=" expression
                      <defer: $main::VARIABLE{$item{VARIABLE}} = $item
+{expression} >
# ...
[download]

Enter --> a=1

Enter --> a=2 print a blah blah

       ERROR (line 1): Invalid instruction: Was expecting assign instr
+uction,
                       or print instruction
Error returned by parsing

Enter --> print a
1
[download]

Though perhaps it's laughable because lacks serious studying about how it really must be built.