#!/usr/bin/perl
use warnings;
use strict;

use Data::Diver qw{ Dive };

my $external_structure = { ... };

my %format = (product_id     => ['id'],
              price_excl_vat => [qw[ data price ]],
              unit_qty       => [qw[ data unit_info qty ]]);  # (1)

my $internal_structure = {};
for my $key (keys %format) {
    $internal_structure->{$key}
        = Dive($external_structure, @{ $format{$key} });
}

use Data::Dumper; print Dumper $internal_structure;