my @list = (
'Tomate',
'Kartoffel',
'Kartoffel',
'Tomate',
'rote Paprika',
'Blumenkohl',
'Tomate',
);

my %seen;
$seen{$_}++ for @list;

my @single = grep{ $seen{$_} == 1 }keys %seen;
print $_,"\n" for @single;