../cgi-bin/script.pl?column3=1&column2=2&column1=3

##</code><code>##

my $q = new CGI;
my $href = $q->Vars;

##</code><code>##

my $col_string = join( ',', sort {$$href{$a} <=> $$href{$b}} keys %$href );

##</code><code>##

$col_string =~ s/^,+//;      # remove leading commas
$col_string =~ s/,+$//;      # remove trailing commas
$col_string =~ s/,{2,}/,/g;  # reduce adjacent commas to ","

##</code><code>##

my $col_string = '';
for my $var ( sort {$$href{$a} <=> $$href{$b}} keys %$href ) {
    $col_string .= "$var,";
}
chop $col_string;  # remove final comma

##</code><code>##

my $sql = "SELECT $col_string FROM my_table";