I would just have used map to make that a lot more simple:

print map { "$_|" } @my_list
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And a design pattern that you should start to recognize is that as soon as you see the concatenation operator, you should be thinking about changing that to a list. So

$my_id_list .= $my_list[$i]."|";
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should change to

push ( @my_id, "$my_list[$i]|" );
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Later, after you're done with the loop, you can join all of the list elements together.

And if your code also removes the last "|" from the strong that you're building, then you can do

push ( @my_id, $my_list[$i] );
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during the loop and

join('|',@my_id);
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at the end of the loop to get the same result.