I think a good solution would be to sort the terms that you're matching for and create a regexp of the strings in sorted order. Sort them so that the regexp tries to match the longest string first, then moves on down in length until it's trying to match the shortest one.

Something like the following should work:

my @terms = ('lymph', 'lymph node');
my @text = ('right lymph node', 'lymph fluid');

# create a regexp that will match the longest
# string first and capture the string that matched
my $words = '\b(' .
    join('|',
    sort { length $b <=> length $a } @terms) .
    ')\b';

for my $text (@text) {
    if ($text =~ /$words/) {
        print $text, ": matched => ", $1, "\n";
    }
}
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