DECC$features question

I have created a callable Perl routine.  I need to have the DECC$feature
DECC$FILENAME_UNIX_REPORT enabled for Excel::Writer::XLSX to for proper
functioning.

Why does calling DECC$FEATURE_SET to establish DECC$FILENAME_UNIX_REPORT
not cause Perl to use/see this feature setting but the definition of the 
DECC$FILENAME_UNIX_REPORT logical does?

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