It's a bug. You get the same result if you run the while_until.pl example in the IO::Prompt distribution with use strict.

The problem is that a next unless defined has to be added in the _get_prompt routine at this point (line 91):

     for (my $i = 0 ; $i < @data ; $i++) {
         local *_ = \($data[$i]);
+        next unless defined;
         if (ref eq 'HASH') {
             splice @data, $i + 1, 0, %$_;
         }
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The for loop removes arguments supplied to prompt by undef-ing them in the array @data, but the index $i still only advances by one. When an option with an argument is encountered, both the option switch and the argument are undef-ed, but the loop will still inspect the position occupied by the option's argument.