The p-values are very small (around 1.6e-12) so I wonder if this ins't some sort of precision issue.
It appears to be a limitation of the tprob function in Statistics::Distributions. The approximation isn't good enough for such small values.

In reply to Re: strange collision when calculating p-values using Statistics::TTest by ReturnOfThelonious
in thread strange collision when calculating p-values using Statistics::TTest by roadnottaken

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