I need to create a big string out of a complex structure and the runtime is way too long.
Back in the Java times I solved similar problems using StringBuffer or StringBuilder which greatly reduce the number of intermediate String objects needed.
Is there something similar in Raku?
Native string arrays pretty much allow you to do that.
my str @parts;
@parts.push("foo")
...
say @parts.join;
Is that what you're looking for?
Additionally: on the MoarVM backend, when you concat strings, they are not actually concatenated in memory, but rather just "linked" together into a single virtual string. Unfortunately, the moment you want to do a regular expression on a string, if does need to be flattened. Which is one of the reasons regular expressions are relatively expensive.