Here are some other ways to do it, including one that doesn't work and one that almost works...

local $_= "Up stop 'Oop' tip stoop put\nup, tops steep 'creap' sleep s
+oap!";
my @words;
push @words, $1
            # Misses "up" if first word in string:
#   while  /(\b\w*(?<=(.))(?!\2)[aeiou]p\b)/gi;
            # Would work if (?<=...|...) were smarter:
#   while  /(\b\w*(?<=^|(.)(?!\2))[aeiou]p\b)/gsi;
            # How to work around (?<=...|...) being dumb:
#   while  /(\b\w*(?:(?<=^)|(?<=(.)(?!\2)))[aeiou]p\b)/gsi;
            # (?<=^) can be shortened to just ^:
    while  /(\b\w*(?:^|(?<=(.)(?!\2)))[aeiou]p\b)/gsi;
            # Or just skip the complex check for 2-letter words:
#   while  /(\b(?:\w*(?<=(.))(?!\2))?[aeiou]p\b)/gsi;
print "( @words )\n";
__END__
( Up stop tip up creap soap )
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