The following works fine. You probably tried something like this yourself, but without the test for an empty field.

use strict;
use warnings;

while (<DATA>) {
    chomp;
    s/,/\./;
    $_ *= 2 if $_;
    s/\./,/;
    print "$_\n";
}
__DATA__
1,
2,22

3,345
[download]

EDIT: I may have misread your post. If you were looking for a neat line of code to replace the above, I'm afraid there probably isn't one.

EDIT:
japhy: Your link looks promising for providing the simplest solution, but doesn't really explain how. A short piece of code might be nice for us stupid people.

sh1tn: I may be missing something here, but don't numbers usually have only one decimal point?