I don't understand what could be the uses of lvalue subroutines? What is it that I can't accomplish with normal subroutines? Could you please post some examples?
LValues are recommended to be avoided. They're fun and all, but they confuse new users who make assumptions about how all subs work, and thus it should be avoided in code when a better way is available.
Sometimes it can be extremely practical to use an LValue result to do dirty work.
substr( $string, $start, $stop ) = 'somevalue' # replaces the specified part of the substring.
However, this also does the same thing:
substr( $string, $start, $stop , 'somevalue' );
The difference here being mostly the former has significantly more expressive power, as it delegates the behaviour to the code that uses it instead of the function itself.
substr( $string, $start, $stop ) =~ s/a/b/g
Would require you to do
my $x = substr( $string, $start, $stop ); $x =~ s/a/b/g/; substr( $string, $start, $stop, $x );
Which is a bit nasty.
Although, there's an additional nasty here, because you have to think about whether doing that substitution or not with $x works retroactively to modify the string or not ( due to substr being an LValue sub). I don't think it does, but Its ambiguous enough I have to check the manual to remember whether it does or not, and that's bad.