I don't know about a regex, but Text::CSV_XS can do it:

use strict;
use Text::CSV_XS;
use Data::Dumper;

my $csv = Text::CSV_XS->new({sep_char=>' '});

$csv->parse('one "two three" four five "six seven eight" nine');

my @columns = $csv->fields();

print Dumper(@columns);
[download]

The result is:

$VAR1 = 'one';
$VAR2 = 'two three';
$VAR3 = 'four';
$VAR4 = 'five';
$VAR5 = 'six seven eight';
$VAR6 = 'nine';
[download]