At the time it returns undef, I examined $!
You can only trust $! to contain something sensible immediately after a failed system call. (Which is something else than a call to system). Since cwd() calls pwd inside backticks, there's no system call, let alone a failed one (unless the fork() would fail, but then, time has passed). You may have more success in checking $?.

In reply to Re: backticks and 'Cannot allocate memory' by JavaFan
in thread backticks and 'Cannot allocate memory' by NetWallah

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