Declare your format variables with either vars or our and then within the loop use local e.g

use vars qw($sku, $description, $price, $qty);

foreach my $item (@items) {
    local($sku, $description, $price, $qty) = @$item;
    write(PURCHASEORDER);
}
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Or you could fully qualify your variables in your format and localize the appropriate variables i.e

use strict;

format STDOUT =
@<<<<<<<  
$::foo
.

local $::foo = 'a string';
write STDOUT;

__output__

a string
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Or you can declare the lexicals before the format, if that's feasible. See. perlform for more info.

update: added second to last sentence and finished the incomplete middle sentence