I very recently encountered a spurious(?) error message (on v5.8.2):

Use of uninitialized value in substitution (s///) at (eval 2) line 21
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Which resulted from the addition of one line like this:

   $hash{$var1}{'string'} = $var2;
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I confirmed that the variables were defined, and that the hash entry was created. I then tried:

   { no warnings 'uninitialized'; 
     # and later:  no warnings;
     $hash{$var1}{'string'} = $var2;
   }
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To no avail. Curiously, the error wasn't occurring at that line (confirmed with print statements); but it disappeared if I commented out that line.

Removing -w got rid of the error.