As others have said, placeholders are for data -- table names are not data. Kudos, though, for thinking about placeholders and the implication that you care about safety in DBI-based scripts!

What you want in this case is quote_identifier to properly quote the table name, then inline that into a do:

sub drop {
    my ($dbh, $table_name) = @_;
    my $quoted_name = $dbh->quote_identifier($table_name);
    $dbh->do("DROP TABLE $quoted_name");
}
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This would be called as drop($dbh,'perl'); to drop a table named 'perl'.