$sth->fetch returns an array reference, and $sth->fetchrow returns a list. Each item in the array or list is a column in the SELECT statement. So:

$sth = $dbh->prepare("select a, b, c from foo");
$sth->execute;
while($row = $sth->fetch) {
    # $row->[0] is column 'a'
    # $row->[1] is column 'b', 
    # etc...
}
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and

while(($one, $two, $three) = $sth->fetchrow_array) {
    # $one, $two and $three are columns a, b and c
}
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Michael