$ perl -E'sub f()  { 0 }  say f + 1;'
1

$ perl -E'sub f($) { 0 }  say f + 1;'
0
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$ perl -E'sub f    { say $_[0]; }  f { foo => "bar" };'
HASH(0x816c158)

$ perl -E'sub f(&) { say $_[0]; }  f { foo => "bar" };'
CODE(0x817bc80)
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$ perl -E'         $x = foo; say $x; sub foo { "sub" }'
foo

$ perl -E'sub foo; $x = foo; say $x; sub foo { "sub" }'
sub
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Update: Added an example to show that prototypes aren't the only source.