What is the difference between using curly-brackets there and square-brackets?

In this context, curly brackets are the anonymous hash 'composer' (to use the terminology of perlref), square brackets are the anonymous array composer. See Making References items 3 and 2, respectively, in perlref.

Update: Maybe one should say "the anonymous { hash => 'composer' } and the anonymous [ qw(array composer) ]".

In reply to Re^3: Perl Hashes, keys under keys (I think)? by AnomalousMonk
in thread Perl Hashes, keys under keys (I think)? by mmartin

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