Not quite what you asked, but the shortest variant would be

f;
[download]

in which case the variable would be autovivified from an undefined value.  Of course, this only works (without warnings) as long as you don't read $ref or $$ref before having assigned it something...

#!/usr/bin/perl -l
use strict;
use warnings;

sub f {
    my $ref=shift;
    $$ref=4711;
    print "$ref -> $$ref";
}

f;

__END__
$ ./833010.pl
SCALAR(0x62d238) -> 4711
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P.S.: same thing happens in dk's suggestion, btw, when taking a reference to the return value of f (i.e. \f).