#! /usr/bin/perl
use strict;
use warnings;


my %data;

while ( my $line = <DATA> ) {
    chomp $line;
    my ( $id, $date, $value ) = split /;\s+/, $line;
    # store first occurence of $value with the data
    $data{$value} = $date       if !exists $data{$value};
}

# kontrolle
use Data::Dumper;
print Dumper \%data;


__DATA__
ID; Datum; VeränderbareWerte
01; 2011-01-01; abc
02; 2011-01-01; jkl
01; 2011-01-02; abc
01; 2011-01-03; xyz