DBD::Oracle doesn't come right out and say so, it does restrict the types you can bind. From the driver code (dbdimp.c),

static int
oratype_bind_ok(dbtype)    /* It's a type we support for placeholders 
+*/
    int dbtype;
{
    /* basically we support types that can be returned as strings */
    switch(dbtype) {
    case  1:    /* VARCHAR2    */
    case  5:    /* STRING    */
    case  8:    /* LONG        */
    case 23:    /* RAW        */
    case 24:    /* LONG RAW    */
    case 96:    /* CHAR        */
    case 97:    /* CHARZ    */
    case 106:    /* MLSLABEL    */
    case 102:    /* SQLT_CUR    OCI 7 cursor variable    */
    case 112:    /* SQLT_CLOB / long    */
    case 113:    /* SQLT_BLOB / long    */
    case 116:    /* SQLT_RSET    OCI 8 cursor variable    */
    return 1;
    }
    return 0;
}
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I'm guessing that NUMBER isn't on this list because Oracle's numbers are in a funky, multiple-byte packed fixed-point format. They won't necessarily fit into any of Perl's native types.

Bind a string instead. It'll be automagically converted.