Hi Scotmonk,

This is a great use-case for using an ordered hash (like a hash but with key insertion order preserved). Fortunately, such a module exists. For this demonstration, I'm more interested in the hash keys, not the hash values (i.e. hash key-driven implementation).

# https://www.perlmonks.org/?node_id=11109163

use strict;
use warnings;
use feature 'say';

use Hash::Ordered;

tie my %elems, 'Hash::Ordered';
tie my %match, 'Hash::Ordered';
tie my %nomatch, 'Hash::Ordered';

my $skip_duplicates = 0;  # set to 1 to skip duplicates
my $num_elements = 13;    # number of elements to read
my $num_read = 0;         # number of elements read

my @lines = <DATA>;
chomp @lines;

# read elements

while ( @lines && $num_read < $num_elements ) {
    my $line = shift @lines;
    foreach my $elem ( split / /, $line ) {
        if ( $skip_duplicates ) {
            $num_read++ unless exists $elems{ $elem };
        } else {
            $num_read++;
        }
        $elems{ $elem } = undef;
        last if $num_read == $num_elements;
    }
}

say "data elements";
say join(' ', keys %elems);

# matched, not matched

if ( @lines ) {
    foreach my $elem ( split / /, shift @lines ) {
        ( exists $elems{ $elem } )
            ? $match{ $elem } = undef
            : $nomatch{ $elem } = undef;
    }
    say "match";
    say join(' ', keys %match);
    say "nomatch";
    say join(' ', keys %nomatch);
}
else {
    say "no more lines";
}

__DATA__
1 2 6 4 5
6 7 8 9 10
1 2 11 12 13
6 14 15 16 17
2 18 19 20 21
[download]

Output: $skip_duplicates = 0

data elements
1 2 6 4 5 7 8 9 10 11
match
6
nomatch
14 15 16 17
[download]

Output: $skip_duplicates = 1

data elements
1 2 6 4 5 7 8 9 10 11 12 13 14
match
2
nomatch
18 19 20 21
[download]

Regards, Mario