You can pass the whole hash by reference, then deref what you want using a hash slice if that's what you mean. Otherwise, what's the point if it's all by reference?

(tested on 5.22.1) See perldata for more info.

use strict;
use warnings;
use Data::Dumper;

sub slice_by_ref {
  my $h = shift;
  my %subset = %$h{'foo', 'bar'};
  print Data::Dumper::Dumper(\%subset);
}

my $h = {blonk => 2, foo => 3, squink => 5, bar => 8};
slice_by_ref($h);
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OUTPUT:

$VAR1 = {
          'foo' => 3,
          'bar' => 8
        };
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