my $sql;
{ 
  my @values = ('?') x @columns; # I love that, BTW...
  local $" = ',';
  $sql = "INSERT INTO $table (@columns) VALUES (@values)";
}
$dbh->prepare($sql) or die("Couldn't prepare $sql " . $dbh->errstr);
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Sure, the likelyhood of any of the DBI/DBD stuff using a string-interpolated array is pretty low but it is still bad practice.

--
$you = new YOU;
honk() if $you->love(perl)