I tried to golf this down and got the following on Parrot (it runs on Moar and JVM):
$ perl6-p -e 'Proxy.new( FETCH => sub ($var) { 42 } );'
Segmentation fault
The next three commands are okay:
$ perl6-p -e 'Proxy.new( FETCH => sub ($) { 42 } ); say "alive"'
alive
$ perl6-p -e 'Proxy.new( FETCH => method { 42 } ); say "alive"'
alive
$ perl6-p -e 'Proxy.new( FETCH => { 42 } ); say "alive"'
alive
