Perhaps the following will be helpful:

use strict;
use warnings;

my %h;
my $HoA = {
    'music'       => [ 9, 53, 3,  15 ],
    'conferences' => [ 7, 56, 15, 53 ],
};

@h{@$_} = undef for values %$HoA;
my @ids = keys %h;

print "@ids";

# 53 56 3 7 9 15
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What is this @h{@$_} = undef notation? See Hash from two arrays.