The in you see in jeffa's post is a filename.. you can actually use any number of input filenames as in
perl -F'\W+' -ane 'print join $/,@F' in1 in2 in3 in4 in5 in6 in7 >> wordlist
Personally I would amend this to read
perl -F'\W+' -alne '@word{@F}=(); END { print sort keys %word }' in >> wordlist
which outputs only unique words, sorted alphanumerically.