Note that:

    my $s = ();
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is equivelent to

    my $s = (undef);
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which is equivelent to:

    my $s;
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The first use of a hash ref autovivifies the referred hash making the previous assignment redundant (and misleading). Consider:

use strict;
use warnings;
use Data::Dump::Streamer;

my $s = ();
Dump ($s);
$s->{'a'} = 10 ;
Dump ($s);
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which prints:

$VAR1 = undef;
$HASH1 = { a => 10 };
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and note that the contents of $s changes from an undefined value to a hash ref.

If you really want to indicate intent with an assignment use:

    my $s = {};
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