I have used global variables in one form or another for years, and I have been constantly admonished that it is a bad thing to do. Could someone please provide a cogent argument why?

Surprised I don't have a list of references on this topic yet. Here's a start. Other cool references welcome.

References

• Action at a distance (wikipedia) - an anti-pattern in computer science in which behavior in one part of a program varies wildly based on difficult or impossible to identify operations in another part of the program. I see Perl gets a dishonourable mention for its now deprecated $[ variable (the principle is that a declaration in one part of the program shouldn't drastically and invisibly alter the behavior of some other part of the program).
• Global Variables Are Bad (wiki.c2.com)
• Why are global variables evil? (SO)
• Why is Global State so Evil? (stack exchange)

From On Coding Standards and Code Reviews:

• Information hiding: Minimize the exposure of implementation details; provide stable interfaces to protect the remainder of the program from the details of the implementation (which are likely to change). Don't just provide full access to the data used in the implementation. Minimize the use of global data. Avoid Action at a distance.

From On Interfaces and APIs:

• Before lexical file handles were introduced in Perl 5.6, those evil old global file handles spawned a host of unfortunate idioms, such as: select((select($fh), $|=1)[0]). In terms of a clear and simple interface to perform a simple task (set autoflush on a file handle), it doesn't get much worse than that, a compelling illustration of why keeping state in global variables is just plain evil. Thankfully, nowadays you can replace it with: $fh->autoflush().

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