#!/usr/bin/perl
use warnings;
use strict;
use Text::CSV;
use Data::Dumper;

my %hash;
my $csv = Text::CSV->new( { binary => 1 } )
  or die "can't use CSV" . Text::CSV->error_diag();

while ( my $row = $csv->getline(*DATA) ) {
    push @{ $hash{ shift @$row } }, @$row;   # as previously mentioned by Eily
}
$csv->eof or $csv->error_diag();

print Dumper \%hash;

 __DATA__
 abc,2,3,4,5,6 
 abc,7,5,2,1,6,2,3 
 abc,8,2,1,3,1,4
 def,8,9,4,5,6 
 def,5,6,2,1