#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %idx;
chomp and push @{$idx{substr $_, 0, 1}}, $_ for <DATA>;
print Dumper \%idx;

__DATA__
Anaconda
Aachen
Armenien
Apple
Berlin
Bär
Brombeeren
Bruch
Charite
Charlie