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Surname L20, First Name L20, Town L20 
-------------------------------------------------- 
...
Sarte        Jean Paul       Montmarte 
Rikard       Frank           Amsterdam 
Rodin        Paul            Montmarte

Town         Surname       Firstname

...
Montmarte    Sarte         Jean Paul  
Montmarte    Rodin         Paul 
Amsterdam    Rikard        Frank

my $data = "clients.dat";
open (DAT, $data) or die "\"$data\" not existed or can't be opened!\n"
...
        die "Invalid town name supplied: $a";
}