I'm not sure if it's exactly what you were looking for, but I used the built-in perl function grep to examine my list for entries that are already in there. Here's the snippet of code I used to test the grep function:
# Create an initial list
@list = (9,2,3,4,5,6);

# Data we wish to insert
@insert_data = 4..9;

# For every datum that we wish to
# insert, check for it's existance
# in the list and append it if it is
# not found
foreach $item (@insert_data){
    next if grep /$item/, @list;
    push @list, $item;
}

# Print out our final list
foreach $item (@list){
    print $item, ",";
}

[download]

This results in the output: 9,2,3,4,5,6,7,8.

Hope this helps -- Rolly

Update: See comments below for reasons why this isn't an efficient solution.

In reply to Re: best data structure by RollyGuy
in thread best data structure by Anonymous Monk

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