Why do mathematicians generally write definitions in “declarative” rather than “imperative style? In programming, we can make the distinction between declarative / functional and procedural / imperative programming. The distinction is not exact, but nevertheless meaningful.
One major difference is that declarative style tends to make use of recursion in places where imperative style makes use of iteration.
Consider the following (informal) imperative style definition of a subset $X\subset \mathbb N$ given a function $f:\mathbb N \to \mathcal P( \mathbb N)$.

Put $0$ in $X$.
Repeat to infinity: For each $x\in X$, put the elements of $f(x)$ in $X$.

This means that $X$ is not an immutable object, but more akin to a “list” in imperative programming languages. I understand that mathematicians would call $X$ “ill-defined”, but as an imperative style program, this is (the pseudocode of) a perfectly well-defined program.
In general, I know that mathematicians don’t like “mutable” objects, and maybe this is the reason why they don’t like such imperative-style definitions. But I don’t really understand why.
Why do mathematicians consider such “imperative” style definitions bad, or even ill-defined, given that they can be quite intuitive and also well defined as an imperative program?
 A: There's also, I think, a readability concern here.
Mathematical proofs are often (usually?) not read in a linear fashion: the reader jumps around to first get a sense of the "global" structure of the argument, then dives into some details, then more details, and so forth. For example, I often read even very short proofs backwards, starting by identifying what sort of thing happens at the conclusion and then unwinding the work that goes into it. 
Granting that this is something that happens, it's important for an object defined at the beginning of the argument to have the same meaning at the end of the argument for the benefit of the reader. In conjunction with the fact that "imperative" definitions can be rephrased as "declarative" ones without significant difficulty (per the second half of Carl Mummert's comment) I think this presents a significant advantage.
Of course this raises the question of why the same argument doesn't apply to programming as well - why isn't the declarative approach also more intuitive there? I'm not an experienced programmer (to put it mildly), but I'll hazard the following guess: a program, unlike a proof, is describing a process which takes place over time. This means that there is much more emphasis on reading in order (at least locally). So the difference, with respect to this concern, is that the "static" nature of mathematical proofs makes reading out of order a valid strategy, while the "dynamic" nature of programs makes that not as good an idea.
A: I cannot speak for all mathematicians, but one key difference in computer science versus mathematics is that objects in computer science are typically viewed as mutable (unless they are declared 'const' or similar), while in mathematics we use names to refer to specific mathematical objects that cannot change. The difference happens because computer scientists are typically thinking of how variables work in programming, while mathematicians only use variables as names for mathematical objects. 
In the style of exposition from computer science, this makes sense:


*

*Define an empty list, $L$,

*Push '1' onto the left of $L$,

*Push '2' onto the left of $L$,

*Now $L = \langle 2,1\rangle$.


Here the actual contents of the list $L$ changed twice during the instructions.  We are thinking of the instructions as a kind of pseudocode. 
In a common style of mathematical exposition, if we say "Let $L$ be the empty list", we mean that $L$ is the empty list. If we then say "put '1' on the left end of $L$", we obtain a new list, not $L$, because we already defined $L$ to be the empty list.  We could say "Redefine $L$ to be the list obtained by push '1' on the left end of the current list $L$", but this is verbose.
However, the imperative style used above is completely possible in ordinary mathematical exposition. It would read like this:


*

*Let $X_0=\{0\}$.

*For each $i \in \mathbb{N}$, let $X_{i+1} = \displaystyle\bigcup_{x\in X_i} f(x) $.

*Let $X = \bigcup_{i \in \mathbb{N}} X_i$.


Notice that we didn't "put things in" to an existing set in the sense of redefining a variable to stand for a new set after the variable was defined. Instead, we used different sets $X_i$ for each $i \in \mathbb{N}$. 
At the same time, it is also perfectly normal to say something like "Make a $\{0,1\}$ matrix $M$ by putting in one column for each 8-bit binary number from $0$ to $255$ in lexicographic order". The key here is the "putting in" does not mean redefining $M$ at each step, it only says how to build the matrix $M$. 
