TMTOWTDI sometimes not even involving (much) perl; if the inbound data were JSON I'd look at jq.

#!/usr/bin/env perl
use 5.018;
use JSON::XS ();

my $json = JSON::XS->new->pretty(1)->ascii(1);

my $external_structure = {
    'name' => 'Test product',
    'id'   => 'ABC123',
    'data' => {
        'enabled'     => '1',
        'internal_id' => '',
        'urls'        => { 'main_url' => '' },
        'price'       => '1000',
        'unit'        => 'ST',
        'unit_info'   => {
            'qty'  => '1',
            'gtin' => '1234567890'
        }
    }
};

say $json->encode($external_structure);

exit 0;

__END__

$ perl splonk.plx | 
    jq '{ product_id: .id, price_excl_vat: .data.price, unit_qty: .dat
+a.unit_info.qty }'
{
  "product_id": "ABC123",
  "price_excl_vat": "1000",
  "unit_qty": "1"
}
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