Easy way

Won't work - %Config::Config is readonly.

You can, however, get around that problem using 'tied'. One way is to create a module, let's call it Config_m.pm:

package Config_m;
use Config;

my $tied = tied %Config;
$tied->{mydomain} = 'new value';

1;
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Then load that module into the script:

use Config_m;
use Config;

print $Config{mydomain};
# outputs 'new value'
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This is tha approach taken by ExtUtils::FakeConfig.

Update: Here's a much simpler example, one that doesn't require a separate module:

use Config;
$obj = tied %Config;
$obj->{mydomain} = 'new value';

print $Config{mydomain}, "\n";

#outputs 'new value'
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(I knew there was a far more straightforward example than the way I originally presented it, but a series of brainfarts prevented me from finding it.)

Cheers,
Rob