that code is broken , it uses symbolic references

you probably meant

#!/usr/bin/perl --
use strict; use warnings;
my $rtag = 'WHAT';
my $pp = { FEATURES => { WHAT => {} } };
my $pdi = { 1,1,2,2,3,3 };
@{
    $pp->{'FEATURES'}{$rtag}
}
{
    keys(%$pdi)
} = values(%$pdi);
use Data::Dump;
dd $pp;
dd $pdi;
__END__
{ FEATURES => { WHAT => { 1 => 1, 2 => 2, 3 => 3 } } }
{ 1 => 1, 2 => 2, 3 => 3 }
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and 5.8.8 is very old