Well, then you'll probably want to get the list of column names via a separate query, store those in an array (e.g. "@colnames", then use the array to define the data query and then load the hash:

my $sqlstr = "select " . join(",", @colnames) . " from sometable ...";
my $sth = $dbh->prepare( $sqlstr );

while ( $rowref = $sth->fetchrow_arrayref ) {
    @rowhash{@colnames} = @$rowref;
    ...
}
[download]

(untested, but it should give you a good-enough idea...; um, especially after the update to use "@$rowref" in the hash slice assignment instead of "$$rowref" which was wrong.)

Since you're using mysql, you could get the list of column names by a query like "describe sometable" -- the column name will be the first column in each returned row. (There are other ways, but this is pretty simple.)