In this case join is clearly more indicated however, sometimes I construct csv strings by appending to it (conditionally) values like "xxx," then "yyy," repeatedly. (As you cannot know which one will be the last, you cannot afford not to have the commas after each one.)

In this case, a simple chop after will do the trick:

[...]
chop($param_string);
print "\n$param_string\n";
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Cheers