Why would you need to know whether a vowel is stressed or not? It only depends on the length of the vowel. All vowels other than "e" at the end of a word are either long, accented, or part of a diphthong ("tweeklank"). A long "e" is always written as "ee" at the end of a word, so "e" is always either short, or accented, or part of a diphthong, and never needs an apostrophe.

Something like this (untested!)

if ($name =~ /[^aeiou][aiouy]$/) {
   $possessive = $name . "'s";
} elsif ($name =~ /(s|z|x|sj|sch)$/) {
   $possessive = $name . "'";
} else }
   $possessive = $name . "s";
}
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should work for all Dutch (native) names. You can't do thing correctly for foreign names (such as Dutroux), as they are not spelled according to Dutch rules.