Mario is the only Perl programmer I know who has used dualvar in production code

Well now you know two, because I delight in using dualvars for XS constants :-)

use Fluent::LibFluentBit qw( FLB_LIB_NO_CONFIG_MAP );
say FLB_LIB_NO_CONFIG_MAP;
say 0+FLB_LIB_NO_CONFIG_MAP;
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Output:

FLB_LIB_NO_CONFIG_MAP
2
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The great part is that when you pass them to XS functions, they pass the integer that XS is looking for, and when you dump those parameters to STDOUT or a logger, you get the constant name so you can understand what is going on.