Because the former interpolates a variable into a string which involves stringising the variable and that fails if the variable is not defined.

The latter is simply copying the contents of a variable that happens to be undef into another variable. No interpretation of the contents of the variable required so no error.

DWIM is Perl's answer to Gödel

In reply to Re: 'use of uninitialised value' error message by GrandFather
in thread 'use of uninitialised value' error message by Win

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