use strict;
use warnings;
use Text::CSV_XS;

my $csv    = Text::CSV_XS->new({ allow_whitespace => 1 });
my $string = '"", "2.90", "3.00", "3.10", "3.20", "3.30", "3.40", "3.50", ' .
         '"3.60", "3.70", "3.80", "3.90", "4.00", "4.10", "4.20", "5v"';

if ($csv->parse($string))
{
    my @fields = $csv->fields;
    /(\d+\.\d+)/ && ($_ *= 2) for @fields;
    print "$_\t" for @fields;
}
else
{
    $csv->error_diag();
}

##</code><code>##

 0:12 >perl 1707_SoPW.pl
        5.8     6       6.2     6.4     6.6     6.8     7       7.2     7.4
7.6     7.8     8       8.2     8.4     5v
 0:12 >