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In addition to the problems pointed out by BrowserUk and 7stud,

Nope. You win.

Problems with Reference Prototypes

At some level, they're more predictable...But I'm afraid that these, too, may often be more trouble than they're worth.

You see ... (\$) (\@) (\%) ... don't actually say that you must pass in a scalar reference, an array reference, and a hash reference. Rather, they say you must pass in a scalar variable, an array variable, and a hash variable. That means that the compiler insists upon seeing a properly notated variable of the given type, complete with ``$'', ``@'', or ``%'' in that slot ['slot' means the corresponding position in the argument list when you call the subroutine.] You must not use a backslash. The compiler silently supplies the backslash for you. 

So given this sub:

use strict; use warnings; use 5.012; sub do_stuff(\$) { my $sref = shift; say $$sref; } can call the sub like this:
my $str = 'hello'; do_stuff($str); #legal: no slash in position 1 of arg list --output:-- hello

But the prototype prevents you from calling the sub like this:

my $str = 'hello'; do_stuff(\$str); #illegal: slash in position 1 of arg list --output:-- Type of arg 1 to main::do_stuff must be scalar (not single ref constru +ctor)

But the prototype doesn't stop you from doing this:

my $str = \'hello'; do_stuff($str); #legal: no slash in position 1 of arg list --output:-- SCALAR(0x100839858)
...and then you end up with unintended consequences.

In reply to Re^2: Use reference for functions in functions by 7stud
in thread Use reference for functions in functions by Anonymous Monk

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