The output you've posted did not come from the code that you posted.

Running the code you posted, (second code block) under 5.10 and 5.18 I get this output:

The 1 is a whole number
The 1 is a whole number
The 123.1 has nondigits
The 0.1 has nondigits
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I suspect the code you actually ran to get the output you posted included:

my @numbers = qw(1, 1.0, 123.1, 0.1);
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And did not have warnings enabled.

But that doesn't explain all the differences.

Try posting the actual code you run; and the actual output you get.