Good job tracking this down! Yes, taking a reference to an undefined sub does create a "declaration". And a method call to a declared but undefined sub does fail like:

$ perl -we'sub foo; my $obj = bless {}, "main"; $obj->foo()'
Undefined subroutine &main::foo called at -e line 1.
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It doesn't give the same error as

$ perl -we'my $obj = bless {}, "main"; $obj->foo()'
Can't locate object method "foo" via package "main" at -e line 1.
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because the method is located; it's just not defined.