$ftp->get($filename, $local_file);
open my $fh, '<', $local_file or die "Could not open $local_file: $!";
open my $ofh, ">", $local_file_converted or die "Could not open $local_file_converted: $!";

# wenn Text::CSV
use Text::CSV;
my $csv = Text::CSV->new();
my $ocsv = Text::CSV->new({ sep_char => ';' });
# ---

while (my $line = <$fh>) {

# wenn Text::CSV

    my $status  = $csv->parse($line); # ggfs. fehler abfangen
    my @columns = $csv->fields();

    $status = $ocsv->combine(@columns); # ggfs. fehler abfangen
    $line   = $ocsv->string();

# wenn manuell

    $line =~ tr/,/;/;

# ---

    print $ofh $line;
}
close $fh;
close $ofh;