From perldoc DBI (highlighting by me):

$hash_ref = $sth->fetchrow_hashref;

An alternative to `fetchrow_arrayref'. Fetches the 
**next row**
of data and returns it as a reference to a hash
containing field name and field value pairs.  Null
fields are returned as `undef' values in the hash.
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To fetch all rows, use $dbh->selectall_arrayref.

Or in your case, where there is only one column:

my @rows = @{$dbh->selectcol_arrayref($sql)};
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