Is there a reason that you have to split the lines in the first place? That is, if the last four fileds are significant and always lumped together, you shouldn't waste time splitting and joining those fields. Also, you shouldn't loop over <DATA> more than once unless absolutely necessary. It can be expensive if the file is large.

my %seen;
while (<DATA>) {
    /^([^\|]+)\|(.*)/; # split at the first "|"
    my ($key, $value) = ($2,$1);
    next if $seen{$key};
    print "$key: $value\n";
    $seen{$key}++;
}
__DATA__
34|4|45|56|45
45|34|45|00|23
45|34|45|00|27
34|4|456|56|03
36|4|456|56|03
[download]