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($firstLetter)= ($file_name=~ /^(.)/);
push(@{$hash{uc($firstletter)}}, $infoline);

$hash{'A'} -> [ infolineA1, infolineA2 ... ]
$hash{'B'} -> [ infolineB1, infolineB2 ... ]

foreach $letter (sort keys %hash) {
    print $letter;
...
        print $infoline;
    }
}