Cwd has various implementation of the same function for different OSs. It redefines some functions *intentionally* in order to optimize operations on OSs where certain paradigms are supported.

It attempts to disable warnings if they are enabled, but -W (as opposed to -w) forces the warnings to remain on.

From Cwd: 

local $^W = 0;  # assignments trigger 'subroutine redefined' warning
*{$name} = \&{$map->{$name}};

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In reply to Re: perl reports fastcwd 'redefined' when using Cwd and -W by ikegami
in thread perl reports fastcwd 'redefined' when using Cwd and -W by argv

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