AH, damn it I have forgotten everything, I swear.

Don't you have a ~/learnperl/arrays/unique.pl file? You should make several of that kind, here is what I have

$ grep -i "%seen" *pl
cygwin.mingw.path.pl:           my %seen;
pm.911928.pl:    my %seen;
pm.912276.pl:my %seen_header;
pm.912276.pl:my %seen_subheader;
set.export.path.pl:    my %seen = ();

Yes, you could refer to Tutorials/perlintro/perlfaq... http://pleac.sourceforge.net/pleac_perl/arrays.html , but I find I remember these things better when I type the code myself and refer to my files :)