Capture an input? Your terminology makes no sense to me. What do you mean by "capture <stdin>"?

Whatever it is you want to do, if you know how to do it with open3, keep doing it that way. There's nothing special about "fixed.pl" on that side of things.

PS — Please don't use <pre>. At all. Place code, computer text, inputs, outputs, etc in <code></code> or <c></c> tags. And use <p> at the start of paragraphs.