Perhaps also interesting is that lvalues remain lvalues even after passing them to the unary + operator.

sub foo {}

#foo(substr "hello", 10, 1);       # fatal error as you pointed out
#foo(+substr "hello", 10, 1);      # I thought this would be a workaro
+und
foo(scalar substr "hello", 10, 1); # but you need something like this

print "This only gets printed if the last line didn't cause a fatal er
+ror\n";
[download]

And testing to make sure:

perl -e '+$a = "Hello, world!\n"; print $a'
Hello, world!
[download]

(I know it's documented that + does absolutely nothing, and I guess that does mean it doesn't convert lvalues to rvalues, but it's still somewhat surprising to me.)

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