It's one of the documentation pages. 

man perldiag
perldoc perldiag
perl -Mdiagnostics script.pl

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perldiag

Nested named subs are not supported by Perl. For starters, the nested sub wouldn't be private. Secondly, you'll get weird errors. (You're notified if the second can happen by "will not stay shared" warnings.)

In reply to Re^3: Strictly nested sub warnings by ikegami
in thread Strictly nested sub warnings by raybies

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