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perl -ne '$_=lc;s/\W+/ /g;@w{split /\s+/}=();END{$,="\n";print sort ke
+ys %w}'

while (<>) { # for each input line
 $_=lc;      # lowercase
...
}
$,="\n";     # separate 'print' args with a newline
print sort keys %w; # print the sorted keys

tr -cs '[:alnum:]' '\n' < textfile |tr '[:upper:]' '[:lower:]'|sort|un
+iq