Quick and dirty? How about:

my %seen;
my @ABC;
for my $element (@XYZ) {
    next if $seen{$element}++;
    push @ABC, $element;
}
@XYZ = @ABC;
[download]

Of course, it's a huge waste to create the second array for a placeholder (even though that code is very easy to understand.) You probably want to delete the entries in place, using splice or array slices, if the array can possibly be of any length.