You would use a hash of arrays, as shown in perllol:

my %hash;
while ( <FILE> ) {
    chomp;
    my ( $key, $value ) = split /,/ $_;
    # treats hashvalue as reference to an array:
    push @{  $hash{ $key }  } , $value;
}

foreach my $key ( keys %hash ) {
    # same dereferencing as above.
    print "$key," , join( "," => @{  $hash{ $key }  } ) , "\n";
}
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Cheers, Sören