The snippet my $row = $csv->getline ($fh) in the example reads a line from the source file, and intelligently splits it up into fields. The Text::CSV library knows how do do that and takes quotes and the like into account. The $row scalar that it returns is slightly misleading as it is actually an array reference to the fields it extracted.
The next line: s/,/~z~/g for @$row; iterates over each field in the row and performs your substitution.
Question to rahulr: Why do you want to replace commas with "~z~"? It looks like you are trying to escape them. If so what would you do if you encounter a real "~z~" in your input? You might want to re-think how you are processing your data so that escaping works properly.