The Observation

While studying up on Perl to get 'big picture' insights, something hit me. Perl has a bewildering arsenal of subroutine DECLARATION and CALLING syntax options. These options can interact and conflict with each other in complicated ways. In other words, depending on how you organize your code, you can construct code in such a way that your subroutines ...

• MUST be CALLED with commas separating the arguments
• MUST NOT be CALLED with commas separating the arguments
• MAY be CALLED with commas separating the arguments
• MAY be CALLED with a leading ampersand
• MUST NOT be CALLED with a leading ampersand
• MUST be CALLED with a leading ampersand
• MUST be CALLED with parenth around arguments
• MUST NOT be CALLED with parenth around arguments
• MAY be CALLED with parenth around arguments
• MUST be CALLED with a specific number of arguments
• MAY be CALLED with any number of arguments
• MAY be CALLED before it is DECLARED in your code
• MAY be CALLED before it is DECLARED and DEFINED in your code
• MUST NOT be CALLED before it is DECLARED in your code
• MUST NOT be CALLED before it is DECLARED and DEFINED in your code
• MAY be DECLARED using parenth (aka prototype) between sub NAME and CODEBLOCK

The Question

Is there a 'big picture' way to comprehend how these options relate to one another? Is there a synergistic rationality behind it that someone can say in one or two sentences? Is it basically the historical trends of the language that has resulted in this matrix of options? Any insight welcome.

Note: for purposes of this post, the terms "MUST" and "MUST NOT" are defined as "violating this rule will cause a either a warning message under 'use warnings' or an error message, or both."

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