Perl5's grammar is mutable too. 
$ perl -E'sub f()  { 0 }  say f + 1;'
1

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

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

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

[download]
$ perl -E'         $x = foo; say $x; sub foo { "sub" }'
foo

$ perl -E'sub foo; $x = foo; say $x; sub foo { "sub" }'
sub

[download]

Update: Added an example to show that prototypes aren't the only source.

In reply to Re: Perl 6 and Perl 5 parsing by ikegami
in thread Perl 6 and Perl 5 parsing by Anonymous Monk

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