In Perl, what is the difference between accessing an array element using @a[$i] as opposed to using $a[$i]?

by 
$a[...]   # array element

returns the one element identified by the index expression, and

@a[...]   # array slice

returns all the elements identified by the index expression.

As such,

  • You should use $a[EXPR] when you mean to access a single element in order to convey this information to the reader. In fact, you can get a warning if you don’t.
  • You should use @a[LIST] when you mean to access many elements or a variable number of elements.

But that’s not the end of the story. You asked for practical and tricky (subtle?) differences, and there’s one noone mentioned yet: The index expression for an array element is evaluated in scalar context, while the index expression for an array slice is evaluated in list context.

sub f { return @_; }

$a[ f(4,5,6) ]     # Same as $a[3]
@a[ f(4,5,6) ]     # Same as $a[4],$a[5],$a[6]

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