If I understood right, you have a list of words in a string and you want to add a word to that string, the new word should not exist already in the words string, am I right till here? If yes then you should do as follows:

# suppose you have:
$line = qw(advanture door tree);
# and $word is:
$word = 'ad';
...

if ($line =~ /\b$word\b/) {
  next;
} else {
  # add $word to the list
}
# only if $word will be exactly 'adventure' or 'door' it will be skipe
+d over
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'\b' is a word boundry like a space a comma (anything that is not a-z, A-Z, 0-9 or '_'), so the regexp in the 'if' statement is exactly what you need.

Hotshot