Here's another alternative:

$sth->bind_col(1, \my $key);   

my %rows;

while(my $row = $sth->fetchrow_hashref) {
  $rows{ $key } = $row;
}
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With this version you don't need to maintain a list of columns, and it will automatically make the first column the key in %rows. It's probably more efficient/faster to use $sth->fetchrow_hashref than doing a foreach since $row is made using a hash slice.