$ perl -Mstrict -Mwarnings -MO=Deparse -e '
use constant FOO => sub { join( shift, @_, ); };
printf( "%s\n", FOO->( ", ", 1 .. 10, ), );
'
use constant ('FOO', sub {
    BEGIN {${^WARNING_BITS} = "UUUUUUUUUUUU"}
    use strict 'refs';
    join shift @_, @_;
}
);
BEGIN {${^WARNING_BITS} = "UUUUUUUUUUUU"}
use strict 'refs';
printf "%s\n", sub {
    join shift @_, @_;
}
->(', ', 1..10);
-e syntax OK