Here is one way:

    #!/usr/bin/perl -wl

    use strict;

    my @many = (1,4,2,3,2,1);


    my %count;

    $count{$_}++ for @many;

    my @once = grep {$count{$_} == 1} @many;


    print "@many";
    print "@once";

    __END__
    Prints:

    1 2 3 2 1
    4 3
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--
John.