greg@spark:~/test$ cat sleepingsquirrel 
#!/usr/bin/perl

print "$_\n" for sort keys %{{map {$_,()} grep /^[a-z]+$/, (split /\s/, join(" ",<>))}};
greg@spark:~/test$ time sleepingsquirrel Net-HOWTO >words.txt

real    0m0.178s
user    0m0.158s
sys     0m0.016s

greg@spark:~/test$ cat jasper 
#!/usr/bin/perl

$/=undef;
print "$_\n" for sort <> =~ /\b([a-z]+)\b(?!.*\b\1\b)/sg
greg@spark:~/test$ time jasper Net-HOWTO >words2.txt

real    1m8.477s
user    1m8.471s
sys     0m0.003s