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my $key_fields    = [ qw( trade_ref sec_code lot of other fields ) ];
my $copy_fields   = [ qw( item_period other fields) ];
...
# $item is a hashref with some but maybe not all of the
# above fields defined
$audit->{$_} = $item->{$_} || '' for @$copy_fields;

@{$audit}{@$copy_fields} = @{$item}{@$copy_fields};
$audit->{$_} ||= '' for @$copy_fields;

@{$audit}{@$copy_fields} = @{$item}{@$copy_fields} || '';