...
    if ( $sum > 310 ) {
        my $sum2 = "volemd";
        #chomp($sum2);  not needed
        $line =~ s/(.*?)\,(.*?)\,(.*?)\,(.*?)\,(.*?)\,(.*?)/$1\,$2\,$3\,$sum2,$5,$6/g;
    }
    elsif ( $sum == 70 ) {
        my $sum3 = "volemd1";
        #chomp($sum3);  not needed
        $line =~ s/(.*?)\,(.*?)\,(.*?)\,(.*?)\,(.*?)\,(.*?)/$1\,$2\,$3\,$sum3,$5,$6/g;
    }
 ...