Very roughly speaking, a shared variable is implemented internally using a form of tieing. Each thread has its own copy of the tied variable, and the real value is stored in a separate area of shared memory. When you read a variable, the tied code gets called (XS rather than perl code), which locks the real shared value, then copies its value into the thread's private address space. Similary a write to a shared variable involves copying the private value into the shared variable. So in a worst-case scenario:

my $s : shared = 'a' x 10_000;
for (1..100) {
    async { my $l = length $s; sleep }
}
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would consume about 1Mb, with each thread having a cached copy of the string at the point it was accessed.

Nested structures are more complicated than that, but roughly speaking the worst-case isn't as bad as the scalar case above.

Dave.