I'm guessing this will work if you add the parentheses on your df call.

my ($fs_type, $fs_desc, $used, $avail, $fused, $favail) = Filesys::Dis
+kSpace::df( $dir );
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Perl seems to think you're trying to use an indirect method call (like "new Data::Dumper" instead of "Data::Dumper->new").

Since you modify @INC in a BEGIN block, I think you can just use Filesys::DiskSpace instead of using require. If you still want to use require for some reason, you'll have to separately import its df to use it without parens.