To preserve order when reducing the list like that, use List::MoreUtils uniq method.

use List::MoreUtils qw(uniq);

my @a=qw(1 3 4 1 2 3);

print join("-",uniq(@a))."\n";
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Prints

1-3-4-2
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Update:And if you dont't want to install and import an entire module for a simple task like this you could just do what List::MoreUtils does internally:

my %h;
@a=map { $h{$_}++ == 0 ? $_ : () } @a;
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(though personally I'd go for using the module in most cases, it's more self-documenting).