perl -d:DumpTrace=+Number::Format -MNumber::Format -E 'say Number::Format::format_number(-0.00004, 5)'

>>>   -e:1:[__top__]:	say Number::Format::format_number(-0.00004, 5)
>>>   Number/Format.pm:316:[_get_self]:	unless (ref $_:(-4e-05,5)[0] && UNIVERSAL::isa($_:(-4e-05,5)[0], "Number::Format"))