Here are some rules of thumb:

• assignments: if the lvalue (the thing assigned to) is an array (e.g. @aSomeArray = ...) or surrounded in parenthesis (e.g. ($var1, $var2) = ... then the thing on the right of the equal sign is in list context.
• array and list elements: these are implicitly in list context. Thus if @x=(1,2); @y=(3,4) then  @z=(@x, @y) is equivalent to @z=(1,2,3,4) and $z=[@x,@y] is equivalent to $z=[1,2,3,4]
• flow of control constructs: depends on the construct. These need to be learned construct by construct, like grammar in a natural language. The most common example is for and foreach - whatever goes between the parenthesis is in list context, e.g. foreach (list-context-here)
• built-in functions or subroutines - the syntax description in Perl documentation will have the word list for any parameter that is in list context. The most commonly used functions expecting list context include push,pop, sort, grep, map, and join. If the built-in function or subroutine has a prototype defined, parameters in list context will also be represented using @. You can get the prototype of any subroutine using the prototype function. For more about prototypes, see perlsub.
• user defined subroutines - check the documentation or prototype as above.

Best, beth

Update struck out push and pop. Pop needs an actual array, not just a list e.g. pop @x or pop @$y as noted by jwkrahn below. The same applies to the first parameter of push as well. This is because both modify the contents of an address in memory rather than simply process a set of elements in sequence.

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