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if($1 eq 'STOCK NO') {
    $csv = $2;
}
$csv .= ",$2";

if($1 eq 'STOCK NO') {
    $csv = $2;
} else {
    $csv .= ",$2";
}

($1 eq 'STOCK NO') ? ( $csv = $2 ) : ( $csv .= ",$2" );

use strict;
use warnings;
...
}

close($fh);