In addition to the other comments, I recommend new programmers also use diagnostics. Not only does that give you more information about the error, it also gives you suggestions on how to fix it. Here's the output I get when I include that:

Global symbol "$another_variable" requires explicit package name at di
+ag.pl line 6.
Execution of diag.pl aborted due to compilation errors (#1)
    (F) You've said "use strict" or "use strict vars", which indicates
+ 
    that all variables must either be lexically scoped (using "my" or 
+"state"), 
    declared beforehand using "our", or explicitly qualified to say 
    which package the global variable is in (using "::").
    
Uncaught exception from user code:
        Global symbol "$another_variable" requires explicit package na
+me at diag.pl line 6.
Execution of diag.pl aborted due to compilation errors.
 at diag.pl line 6
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