Incorrect uninitialized value messages in sprintf

gistfile1.txt

$ perl -v 

This is perl, v5.10.1 (*) built for x86_64-linux

Copyright 1987-2009, Larry Wall

Perl may be copied only under the terms of either the Artistic License or the
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$ perl -w -E'my $x = "foo";my $y = sprintf "%d $x";'
Use of uninitialized value $x in sprintf at -e line 1.

$ perl -w -E'my $x;my $y = sprintf "%d".($x//"")' 
Use of uninitialized value $x in sprintf at -e line 1.

$ perl -w -E'my $x="%d";my $y = sprintf $x' 
Use of uninitialized value $x in sprintf at -e line 1.