My team was debating the idea of writing a large file using pipe characters to delimit the fields, and I pointed out that our data includes pipe characters. So the tilde was suggested, but I found some of those too.

So the obvious question arises, can I find a printable ASCII character that never appears in our big file?

I'd like to do this with just a single pass through the file. For locating a pipe character, all I had to say was

while (<>) {last if /\|/}
printf "| seen at index %d in %s\n", index($_, '|'), $_;
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Update: Should have mentioned that I'm going to run any executable solutions against a 2 GB file. So efficiency may be an issue.

Later: Thanks for the insights! I applaud almut's choice of an indicator array @seen, updated with $seen[ord $chr]++, which should be more efficient than the hash version $seen{$chr}++.

I think I will try kennethk's solution using \Q, both as written and then with the indicator array, as soon as my daughters are in bed and I can connect to my server again.

(Just a minute, sweetie, Daddy will be right with you ...)

It's true the indicator array doesn't offer a builtin keys function to list the seen items, but a grep should do just fine.