print is your friend :-)
print'ing out debug messages often helps very much, sometimes combined with Data::Dumper:

use Data::Dumper;
print STDERR '@INC: '.Dumper(\@INC)."\n\%INC: ".Dumper(\%INC)."\n";
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I tend to add the use Data::Dumper; in the same line as the debug print when adding short-time prints which get deleted once the problem is solved. This is no good syntax, but saves you from forgetting to remove the use once you don't longer need it.