For what it's worth, I find the expression subheaders a bit confusing, and think that a few straight code examples would help.

Might I suggest the following?

Constructor: Resources->new( ... )

Creates a new resource database object $res, initializes it, and returns the new object, or undef in case of error.

  my $res = Resources->new( $filename ); # Load file data

  my $res = Resources->new(); # New with core defaults only

  my $res = Resources->new('_RES_NODEFAULTS'); # totally empty
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The resource is initialized in one of three possible ways based on the single argument, $resfile, that it can take:

No argument or false value

In this case core resource values (The Resources of Resources) are bound and the object is returned.

A resource file name

First the object is created and initialized as above. Then $res->load($resfile) is called, overwriting and adding any new resources found in that file.

The string '_RES_NODEFAULTS'

This creates a completely empty object, with no bound resources.