You didn't include which line has the error, or whether it was a 'warn' or 'carp', so here are both possible reasons:

For some value of $key, the expression $mc_attr_lookup{$key}->{attrs} is undefined. Try dumping $mc_attr_lookup first to understand and validate its structure.

The Dumper routine expects scalars, and some element in @mc_mth_attrs is undefined. You probably wanted to do print Dumper \@mc_mth_attrs; instead.

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