My first attempt to answer a question :-)
just with the Perl elements I know so far.

use strict;
use warnings;

my $header = <DATA>;
while (my $line = <DATA> )
{
    chomp $line;
    my @array = split /;/, $line, -1;
    my $row_id = shift @array; 
    my @sorted = sort {$b<=>$a} @array;
    my $max = shift @sorted;
    my @index = map {$_ + 1} grep { $array[$_] eq $max } 0..$#array;
    print "Max value in the row $row_id is $max ";
    print "in the column(s) ",  @index > 1 ? join ',', @index : @index
+, ".\n";
}

__DATA__
row_id;value1;value2;value3
id001;10;10;30
id002;30;20;50
id003;30;80;10
id004;30;90;90
id005;50;50;10
[download]

This prints:

Max value in the row id001 is 30 in the column(s) 3.
Max value in the row id002 is 50 in the column(s) 3.
Max value in the row id003 is 80 in the column(s) 2.
Max value in the row id004 is 90 in the column(s) 2,3.
Max value in the row id005 is 50 in the column(s) 1,2.
[download]