in reply to Is modifying the symbol table to redefine subroutines evil?

This approach to modifying the symbol-table after a first call is roughly the technique that we used when we implemented a deprecated module: use deprecated; 
package OldeCrufte;
use deprecated qw(do_hack);  # calling OldeCrufte::do_hack() will carp

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package OldeCrufte;
use deprecated;              # using the OldeCrufte module will carp

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In Section Seekers of Perl Wisdom