perl -ne'$seen{$_}||=(print,1)'

Assuming the print will not fail (and for oneliners, I think it's safe to assume print will print succesfully):

perl -ne'$seen{$_}||=print'
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See? print, like many other commands, returns true on success, which allows you to shorten the shortened shortening by another 4 characters!

Implementing s/seen/s/:

perl -ne'$s{$_}||=print'
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Using symbolic references, assuming no special variable names will be used:

perl -ne'$$_||=print'
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I think Perl 6 should have an alias for print that is a single \W character ;) Would be fun for golfing, and could compensate for the needed whitespace with string concats :)

U28geW91IGNhbiBhbGwgcm90MTMgY
W5kIHBhY2soKS4gQnV0IGRvIHlvdS
ByZWNvZ25pc2UgQmFzZTY0IHdoZW4
geW91IHNlZSBpdD8gIC0tIEp1ZXJk