$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]