So what you're after is the maximum size of a scalar variable? (what other languages would call a string, although perl's scalars also handle numeric data)

Scalars (like all perl datatypes) expand and contract dynamically to fit the amount of data you try to stick in them. I have used large scalars before with no problems. I just ran some quick tests on my system and stuck a 10MB chunk of data into a scalar and perl didn't even break a sweat.