What steps have you undertaken to debug this issue?

What values does $b->[1] and $a->[1] have? How could you find this out yourself?

What does the following code output?

$_= '31,wppra00a0513';
print /.+,/;
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Compare the output with

$_= '31,wppra00a0513';
print /(.+),/;
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Maybe you want to read Regexp Quote-Like-Operators on how m/.../ behaves in list context.

Update: My interpretation of your input data might be off-base. Still, the debugging steps outlined above still hold.