"BEGIN { say 1 }" is parsed as an identifier rather than what it really is.

BEGIN.p6

Perl5/Grammar.pm:340:   proto token term { * }
Perl5/Grammar.pm:944:   proto rule statement_prefix      { * }
Perl5/Grammar.pm:1029:  token identifier { '::'? <.alpha> \w* }
Perl5/Grammar.pm:1730:  rule statement_prefix:sym<BEGIN> { :my $*SHIFT_FROM := '@ARGV'; <sym> <sblock> }
Perl5/Grammar.pm:2102:  token term:sym<statement_prefix> { <statement_prefix> }
Perl5/Grammar.pm:3035:  token term:sym<identifier> {
Perl5/Grammar.pm:3036:      :my $name;
Perl5/Grammar.pm:3037:      :my $*HAS_INDIRECT_OBJ = 0;
Perl5/Grammar.pm:3038:      :my $*ALLOW_IOS_VAR    = 0;
Perl5/Grammar.pm:3039:      :my $*ALLOW_IOS_NAME   = 1;
Perl5/Grammar.pm:3040:      :my $*IN_SPLIT         = 0;
Perl5/Grammar.pm:3041:      :my $*IN_OPEN          = 0;
Perl5/Grammar.pm:3042:      <identifier> { $name := ~$<identifier> }
Perl5/Grammar.pm:3042:      ...

nqp-m.bash

NFA_DEBUG=1 nqp-m -e 'grammar G { token TOP { <a> }; proto token a {*}; token a:sym<foo> { <b> }; token a:sym<indirect> { <?{ say("bad") }> \w+ }; proto token b {*}; token b:sym<foo> { <?{ say("good") }> <sym> } }; say(G.parse("foo"))'

  '0' => [ a:sym<foo> a:sym<indirect>]
  '1' => []
  '2' => []
  '3' => [ b]
  '4' => []
  '5' => [ b:sym<foo>]
  '6' => []
  '7' => []
  '8' => []
  '9' => []
  '10' => []
  '11' => []
  '12' => []
  '13' => []
  '14' => []
  '15' => []
][
  '0' => [ b:sym<foo>]
  '1' => []
  '2' => []
  '3' => []
  '4' => []
  '5' => []
  '6' => []
]good
foo

perl6-m.bash

NFA_DEBUG=1 perl6-m -e 'grammar G { token TOP { <a> }; proto token a {*}; token a:sym<foo> { <b> }; token a:sym<indirect> { \w+ }; proto token b {*}; token b:sym<foo> { <sym> } }; say(G.parse("foo"))'
[
  '0' => [ a:sym<foo> a:sym<indirect>]
  '1' => []
  '2' => []
  '3' => [ b]
  '4' => []
  '5' => []
  '6' => []
  '7' => []
｢foo｣
 a => ｢foo｣

perl6-p.bash

NFA_DEBUG=1 perl6-p -e 'grammar G { token TOP { <a> }; proto token a {*}; token a:sym<foo> { <b> }; token a:sym<indirect> { \w+ }; proto token b {*}; token b:sym<foo> { <sym> } }; say(G.parse("foo"))'
[
  '0' => [ a:sym<foo> a:sym<indirect>]
  '1' => []
  '2' => []
  '3' => [ b]
  '4' => []
  '5' => [ b:sym<foo>]
  '6' => []
  '7' => []
  '8' => []
  '9' => []
  '10' => []
  '11' => []
  '12' => []
  '13' => []
][
  '0' => [ b:sym<foo>]
  '1' => []
  '2' => []
  '3' => []
  '4' => []
  '5' => []
]｢foo｣
 a => ｢foo｣
  b => ｢foo｣
   sym => ｢foo｣