export PERL5LIB = ...path/to/lib

##</code><code>##

use ExtUtils::MakeMaker;

sub MY::post_initialize {
    return "export PERL5LIB = ...path/to/lib";
}

WriteMakefile(
   ...
);

##</code><code>##

# --- MakeMaker post_initialize section:
export PERL5LIB = ...path/to/lib