On non-Windows systems, the initial contents of @INC are hardcoded when Perl is built.
On Windows systems, the initial contents of @INC are derived from the location of the perl executable.

Env var PERL5LIB prepends to @INC. (See perlrun.)
Perl command line option -I prepends to @INC. (See perlrun.)
Perl module lib prepends to @INC.
And @INC can be modified as any other array.