You're missing the point. The 'use' statement does its thing at compile-time. That means that as soon as Perl compiles a 'use lib' it affects @INC. That's as true inside a BEGIN as it is outside a BEGIN. Here let me demonstrate:

BEGIN {
  if (@INC) {
     use lib '/foo';
  } else {
     use lib '/bar';
  }
}
print "INC IS: @INC\n";
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Running this code you can see that the 'if' did not effect the 'use' statements. That's because 'if' is a run-time operator and 'use' is a compile-time statement. See:

$ perl begin.pl
INC IS: /bar /foo /usr/local/lib/perl5/5.6.1/i686-linux /usr/local/lib
+/perl5/5.6.1 /usr/local/lib/perl5/site_perl/5.6.1/i686-linux /usr/loc
+al/lib/perl5/site_perl/5.6.1 /usr/local/lib/perl5/site_perl .
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-sam