equick:

In the CPAN docs, it mentions that you can override values in CPAN/Config.pm by putting a file in $HOME/.cpan/CPAN/MyConfig.pm. Perhaps you can then place a bit of code in MyConfig.pm to detect your login and perform the appropriate initializations.

# MyConfig.pm

# Stuff common to all configurations
$CPAN::Config = {
   'applypatch' => q[],
   # bunch of junk snipped out
};

if ($Config{osname} eq 'foo') {
   $CPAN::Config->{build_dir} = '/work/builds/foo';
}
elsif ($Config{osname} eq 'bar') {
   $CPAN::Config->{build_dir} = '/work/builds/foo';
}

1;
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...roboticus

When your only tool is a hammer, all problems look like your thumb.