To me, the regex is correct. I tried to copy and paste the exact code that you posted under the Perl debugger:

  DB<1> $_ = "PID: , VID: 255, SN: AGM163923J5";

  DB<2> print $_ unless /PID: ,/;

  DB<3>
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So it works as I expected (not printing $_ because of the successful match) and not as your reported result. Could it be that there are really two spaces between the colon after PID and the comma, or perhaps a tab and not a space, or some other special character? A starting point for the investigation might be to try this:

print $_ unless /PID:\s+,/;
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to see if it matches or not.