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my @tocheck;
for my $value (values %$self) {
...
}

return grep { $seen{$_} == @tocheck } keys %seen;

my ($n, %seen);
for my $array (values %$self) {
...
  $n++;
}
return grep { $seen{$_} == $n } keys %seen;

my %seen;
for my $value ( map { ref($_) eq 'ARRAY' ? @$_ : () } values %$self ) 
+{
  $seen{$value}++;
}
return grep { $seen{$_} > 1 } keys %seen;