AFAIK, it's a parser issue (hence the syntax error) caused by the fact that Perl supports two flavours of print(), one with the filehandle and the other without it (in which case it defaults to STDOUT).

When perl "sees" $_->[0], it assumes that the latter flavour is in force, and by the time it arrives to "moo\n" it's expecting to find an operator (e.g. a comma, or a concatenation dot, or whatever). On the other hand, using the block resolves this ambiguity towards the former print() flavour.