Different distrobutions compile in different lists of "default" library paths. Usually as a way of making sure that modules you install with CPAN don't overwrite/get overwritten by modules installed using the distrobutions own packaging system.

If a directory is in your @INC because of the $ENV{PERL5LIB}, then it will also show up in the %ENV section just above @INC. I don't believe anything similar happens for directories added using -I

laptop:~> setenv PERL5LIB /perl5lib/env/var
laptop:~> perl -V -I /dash/I | tail -15
  Built under linux
  Compiled at Apr  4 2004 05:57:53
  %ENV:
    PERL5LIB="/perl5lib/env/var"
  @INC:
    /dash/I
    /perl5lib/env/var
    /usr/local/lib/perl/5.6.1
    /usr/local/share/perl/5.6.1
    /usr/lib/perl5
    /usr/share/perl5
    /usr/lib/perl/5.6.1
    /usr/share/perl/5.6.1
    /usr/local/lib/site_perl
    .
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The Config module tells you all sorts of info on how your perl was built...

laptop:~> perl -l -MConfig -e'while(($k,$v)=each(%Config)) { print "$k
+=$v" if $k =~ /lib/i or $v =~ /lib/i }' | head
archlibexp=/usr/lib/perl/5.6.1
installarchlib=/usr/lib/perl/5.6.1
installprivlib=/usr/share/perl/5.6.1
libpth=/usr/local/lib /lib /usr/lib
libs=-lgdbm -ldb -ldl -lm -lc -lcrypt
privlibexp=/usr/share/perl/5.6.1
archlib=/usr/lib/perl/5.6.1
config_arg11=-Dsitelib=/usr/local/share/perl/5.6.1
config_arg12=-Dsitearch=/usr/local/lib/perl/5.6.1
config_arg21=-Duseshrplib
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