BrowserUk has recently written this article which brought to my knowledge the most interesting thing I've read since I'm at the Monastery. That is, that you can keep (multiple) lvalue refs of substr return values and manipulate them.

Just to be sure, I tried

$ perl -le 'print ref \substr "foo", 1'
LVALUE
[download]

and so... wow! it seems that we really have a LVALUE datatype. Now, that's cool; OTOH we have lvalue-able subs, but what about them?

$ perl -le 'my $u; sub u :lvalue {$u} print ref \u'
SCALAR
[download]

Huh, no, it's just a plain old SCALAR. Which raises this meditation: lvalue subs can be cool, but can I imitate substr's action at distance behaviour to do something potentially more interesting? Well, it turns out that I can:

#!/usr/bin/perl -l

use strict;
use warnings;

package Append;

sub TIESCALAR { bless \$_[1], $_[0] }
sub FETCH { ${${ $_[0] }} }
sub STORE { ${${ $_[0] }} .= $_[1] }

package main;

sub append :lvalue {
    tie my $v, 'Append', \$_[0];
    $v;
}

my $u='Foo';
(append $u)='Bar';
print $u;

__END__
[download]

Incidentally, if I substitute

(append $u)='Bar';
print $u;
[download]

with

print +(append 'Foo')='Bar';
[download]

it still works, whereas I would have expected it to raise an error about an attempt to modify a read-only value. Why doesn't it?

Whatever, this is at best tricky. And this is an oversimpliflied example. It could get too complex in a more serious attempt.

So getting back to the lvref topic hinted to at the beginning of this post, the matter is: the above could be doable in an easier way if

1. lvalue subs were so as to return a blessed reference into the LVALUE package,
2. this package provided a, say, STORE method to override standard assignment,

so that the above may be simply:

sub append :lvalue {
    local *LVALUE::STORE = sub {
        my $self=shift;
        $$self .= $_[0];
    }
    $_[0];
}
[download]

What about this idea? (Or some variation thereof!)

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