The problem is that MooseX::ConfigFromFile (the "parent" role of MooseX::SimpleConfig) is doing some pretty odd stuff with the configfile attribute, such as trying to call the default before an instance has been created (see here for details). What you are trying to do simply won't work because of this. Probably the simplest alternative is to do something like:

#!/usr/bin/perl 
package My::Config;

use Moose;
with 'MooseX::SimpleConfig';

sub new_from_config_shortname {
    my ($class, $filename) = @_;
    $class->new_from_config( configfile => '/some/path/' . $filename .
+ ".yaml" );
}

my $config = My::Config->new_from_config_shortname("config");
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Obviously shorten 'new_from_config_shortname' to something more appropriate, but that should give you what you want.