.can you please get me through how this is working

Your tests are not adequate to understand choroba's suggested code. Try this (demonstrated here under the Perl debugger):

  DB<1> $string = 'Tetsing " code implementationm';

  DB<2> $result = $string =~ tr/"//;

  DB<3> p $result
1
  DB<4> p $string
Tetsing " code implementationm
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In essence, choroba is testing the value that ends up into $result in the code above. And it adds a quote mark at the end of the string if the value returned by tr/// is odd (meaning that quote marks are unbalanced).