This is a proposal of a change in Tcl syntax:

When I want to create a big datastructure (with lists or dicts), I like to write the code in serveral lines for clarity. The problem with this expression: [comand_name arg0 arg1 arg2...] is that backslash are necessary if I want to write that expression in several lines. It's a little bit cumbersome:

set big_data_strucutre [dict \
                          key1 value1 \
                          key2 value2 \
                          key3 [list item1 item2 item3] \
                          key4 value4 \
                          key5 [list item1 [list item21 item22]] \
                          key6 value6]

Maybe it's better to make backslash optional inside square brackets (similar to braces). It's more clear and it's not necessary to type so much:

set big_data_strucutre [dict
                          key1 value1
                          key2 value2
                          key3 [list item1 item2 item3]
                          key4 value4
                          key5 [list item1 [list item21 item22]]
                          key6 value6]