Part 3 of Making References has a good explanation:

# leading +{ disambiguate expression to mean HASH reference
sub hashem {        { @_ } } # silently wrong
sub hashem {       +{ @_ } } # ok
sub hashem { return { @_ } } # ok

# leading {; disambiguate expression to mean BLOCK
sub showem {        { @_ } } # ambiguous (currently ok, but may change
+)
sub showem {       {; @_ } } # ok
sub showem { { return @_ } } # ok
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