Backticks are documented to return undef on error
And to further clearify, that's errors regarding the execution of the given program. The OP also mentions getting the return value - a non-zero exit code means (by convention) that the executed program encountered an error. In that case, backticks will not return undef. However, even backticks set $?, so the return value can be found as $? >> 8.

In reply to Re^2: Capture external program return value by JavaFan
in thread Capture external program return value by llancet

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