my ($a, $b) = (10, 20);

sub mysub : lvalue { $_[0] > 0 ? $a : $b }

mysub(2) = 15;
mysub(-1) = 9;

printf "\$a: %d, \$b: %d, mysub(2): %d, mysub(-1): %d\n", 
        $a, $b, mysub(2), mysub(-1);
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the only difference that i could see mattering is the lack of 'return' statements in my version.

Aha. That's it. The explicit return seems to make the sub return the value of the rhs, but the variable is never modified. That's what "experimental" means.

This works as expected:

#!/usr/bin/perl -wl
use strict;

{
    my $count = 0;
    sub counter : lvalue { ++$count }
}

print counter() for 1..5;
print counter() = -5;
print counter() for 1..5;
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