First thought is that this looks like the start of exactly what SQL::Abstract can do... While you said this was in part a perl excercise, you may still want to consider replacing the whole thing or parts (i.e. making your insert() and update() methods wrappers for those in SQL::Abstract)..

As for getting the field names in a table, is it necessary? you could assume that the data passed in has valid field names (otherwise it will just sql error).. But anyways, i believe that this will work pretty DB-independently:

  my $sth = $dbh->prepare("SELECT * FROM $table WHERE 1=0");
  $sth->execute;
  my @cols = @{$sth->{NAME}};
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