substr() is implemented as an Lvalue subroutine, and it's a fatal error when assigned to incorrectly. I think means that when substr() is passed as a parameter, it gets treated like an Lvalue, since it can be assigned back to by modifying @_.

This code crashes on the subroutine call:

use strict;
use warnings;
my $x="";
foo( substr($x,2,1) ); # crashes here
print "Alive!\n"; # not reached
sub foo {}
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This code doesn't:

use strict;
use warnings;
my $x="";
my $y=substr($x,2,1); # warns, doesn't crash
foo($y); # no-op
print "Alive\n"; # we get here just fine
sub foo {}
[download]

I maintain that this is an odd behaviour, one way or the other. My hunch is that it counts as an "oddity" as opposed to a "bug": can the experts confirm or deny this?
--
Ytrew