From perlfaq8:

What's the difference between require and use?

Perl offers several different ways to include code from one file into another. Here are the deltas between the various inclusion constructs:

1) do $file is like eval `cat $file`, except the former
1.1: searches @INC and updates %INC.
1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.

2) require $file is like do $file, except the former
2.1: checks for redundant loading, skipping already loaded files.
2.2: raises an exception on failure to find, compile, or execute $file.

3) require Module is like require "Module.pm", except the former
3.1: translates each "::" into your system's directory separator.
3.2: primes the parser to disambiguate class Module as an indirect object.

4) use Module is like require Module, except the former
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the current one.

In general, you usually want "use" and a proper Perl module.

So the main differences are require works at run-time, use at complile-time and use may import symbols from the module namespace into the current namespace. Does that help?