Thanks for answer. I quickly did a truss to find out who is loading Config.pm and found that it is Test::More who actually loads Config.pm via Test::Builder. It is unfortunate that it gets called on both perl versions but only 5.10 fails :(

my_perl:stat("blib/lib/Config.pm", 0xFFFFFD7FFFDFD640) Err#2 ENOENT
my_perl:stat("/usr/local/pkgs/perl-5.10.0/lib/i86pc-solaris/Config.pm"
+, 0xFFFFFD7FFFDFD640) = 0
my_perl:open("/usr/local/pkgs/perl-5.10.0/lib/i86pc-solaris/Config.pm"
+, O_RDONLY) = 5
my_perl:stat("blib/lib/Config_heavy.pl", 0xFFFFFD7FFFDFD050) Err#2 ENO
+ENT
my_perl:stat("/usr/local/pkgs/perl-5.10.0/lib/i86pc-solaris/Config_hea
+vy.pl", 0xFFFFFD7FFFDFD050) = 0
my_perl:open("/usr/local/pkgs/perl-5.10.0/lib/i86pc-solaris/Config_hea
+vy.pl", O_RDONLY) = 5
vendor_perl:stat64("./Config.pmc", 0x08045DA0)          Err#2 ENOENT
vendor_perl:open64("./Config.pm", O_RDONLY)                     Err#2 
+ENOENT
vendor_perl:stat64("blib/lib/Config.pmc", 0x08045DA0)   Err#2 ENOENT
vendor_perl:open64("blib/lib/Config.pm", O_RDONLY)              Err#2 
+ENOENT
vendor_perl:stat64("/usr/perl5/5.8.4/lib/i86pc-solaris-64int/Config.pm
+c", 0x08045DA0) Err#2 ENOENT
vendor_perl:open64("/usr/perl5/5.8.4/lib/i86pc-solaris-64int/Config.pm
+", O_RDONLY) = 6
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