You don't need (or want) to sort the town names into a separate array; after the AoH is loaded, you just need to loop over the list of sorted array elements, where the sorting is based on the "town" value, like this: 
for my $entry ( sort { $a->{town} cmp $b->{town} } @client ) {
    printf "%s\t%s\t%s\n", $entry->{town}, $entry->{surname}, $entry->
+{firstname};
}

[download]
(not tested)

(updated to add missing "->" in the printf statement, based on fseng's reply below)

In reply to Re^3: How to read data file into an array of hashes by graff
in thread How to read data file into an array of hashes by fseng

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