...
open(my $csv_fh, "<",$Datei_in) or die "...: $!";

my @csv_lines = map {chomp;} <$csv_fh>;  

my $temp = ...; # string in which "Item n" has to be replaced
$temp =~ s/Item (\d+)/$csv_line[$1-1]/ge;  # replace "Item 1" with $csv_line[0]