As far as I understand the debugger, both expressions are evaluated in list context.
The first expression simply returns 1 for success and
I assume 0 an empty list for failure; so it's a single boolean result in list context.
The second expression has capturing parantheses, which (in list context) produces a list of captured results. As your (p)* could not be matched, there is no captured value for (p), so it returns a list with one element: undef.
That's my attempt to explain it. I am sure there are others who can explain it more detailed and accurately and even show some insight into the internals ...
DB<4> x "24-06" =~ /\d+-\d+/
DB<5> x "24-06" =~ /\d+-\d+failure/
- fixed assumption upon failed regex match, added code example, some rephrasing
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