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How is the argument in sleep( 3 ) an lvalue?
In much the same way that \3 is. Not a very good answer, I know, but it's the best I can do. Perl makes @_ an array of aliases to the arguments, as you know. That's much like taking a reference: it requires Perl to consider the arguments in an lvalue context. That aliasing may not happen on builtin functions like sleep, though.

Another example of aliasing is a for loop. It also provides an lvalue context:

use warnings; use strict; my $x; # @$x; # This will die if uncommented for my $foo (@$x) { print "Gar!\n"; }
While I'm certain that I read about the @_ lvalues phenomenon here on PM, I haven't been able to Super Search it up. tye mentions it in Re^5: Warnings not being thrown with DBI (nits).

Caution: Contents may have been coded under pressure.

In reply to Re^3: Autovivification of scalars in sub calls by Roy Johnson
in thread Autovivification of scalars in sub calls by tlm

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