Programming Perl, 4th Edition

The lvalue Attribute

It is possible to return a modifiable scalar value from a subroutine, but only if you declare the subroutine to return an lvalue:

my $val;
sub canmod : lvalue {
    $val;
}
sub nomod {
    $val;
}

canmod() = 5;   # Assigns to $val.
nomod()  = 5;   # ERROR

If you’re passing parameters to an lvalued subroutine, you’ll usually want parentheses to disambiguate what’s being assigned:

canmod $x  = 5;     # assigns 5 to $x first!
canmod 42  = 5;     # can't change a constant; compile–time error
canmod($x) = 5;     # this is ok
canmod(42) = 5;     # and so is this

If you want to be sneaky, you can get around this in the particular case of a subroutine that takes one argument. Declaring the function with a prototype of ($) causes the function to be parsed with the precedence of a named unary operator. Since named unaries have higher precedence than assignment, you no longer need the parentheses. (Whether this is desirable or not is left up to the style police.)

You don’t have to be sneaky in the particular case of a subroutine that allows zero arguments (that is, with a () prototype). Without ambiguity, you can say this:

canmod = 5;

That works because no valid term begins with =. Similarly, lvalued method calls can omit the parentheses when you don’t pass any arguments:

$obj–>canmod = 5;

We promise not to break those two constructs in future versions of Perl 5. They’re handy when you want to wrap object attributes in method calls (so that they can be inherited like method calls but accessed like variables). ...