# änderungen einlesen
open my $cfh, "<", "changes.csv" or die $!;
my %changes;
while (my $line = <$cfh>) {
    my ($aks, $rest) = split m/;/, $line, 2;
    $changes{ $aks } = $rest;
}
close $cfh;

open my $fh, "<", "orig.csv" or die $!;
open my $ofh, "<", "new.csv" or die $!;

while (my $line = <$fh>) {
    my ($aks, $rest) = split m/;/, $line, 2;

    if (exists $changes{ $aks }) {
        $line = "$aks;$changes{ $aks }"; # newline hängt noch dran
    }

    print $ofh $line;
}