my @chars = ('a'..'z', 'A'..'Z',0..9, qw[- _ ! & ? = ] );
sub genFile {
    my( $fh, $mb ) = @_;
    my @buf = ( @chars ) x 61; $#buf = 4096;
    for( 1 .. $mb * 256 ) {
        print $fh join'', shuffle @buf;
    }
}