No idea on WHY the behaviour is different. I'm sure monks with more (perl) guts will come forth.
Args to subs are treated as lvalues (sort of) because they can be modified within the sub, e.g. 
sub f { $_[0] = "abc" }
$s = "1234";
f(substr($s, 0, 1));
# $s is now abc234

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Dave.

In reply to Re^2: "uninitialized value in subroutine exit" warning from CGI.pm by dave_the_m
in thread "uninitialized value in subroutine exit" warning from CGI.pm by dwmcewan

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