When you're not sure about perl syntax, get App::PPI::Dumper

ppi_dumper / wxPPIxregexplain.pl

PPI::Document
  PPI::Statement::Variable
    PPI::Token::Word  	'my'
    PPI::Token::Symbol  	'@refs'
    PPI::Token::Operator  	'='
    PPI::Token::Symbol  	'@allrefs'
    PPI::Structure::Subscript  	[ ... ]
      PPI::Statement::Expression
        PPI::Token::Word  	'sort'
        PPI::Structure::Block  	{ ... }
          PPI::Statement
            PPI::Token::Symbol  	'$a'
            PPI::Token::Operator  	'<=>'
            PPI::Token::Symbol  	'$b'
        PPI::Token::Word  	'values'
        PPI::Token::Symbol  	'%uni_refs'
    PPI::Token::Structure  	';'

perldoc PPI::Structure::Subscript

PPI::Structure::Subscript is the class used for square and curly braces that specify one element of an array or hash (or a slice/subset of an array or hash)

So then [perldoc://subscript], [perldoc://subset], [perldoc://slice] subscript, subset, slice -> perldata#Subscripts, perldata#Slices, perllol#Slices, perlintro#Arrays, perlfunc#delete EXPR,

They are also discussed in the free book (reviewed and recommended by Perl Tutorial Hub) Modern Perl by chromatic a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.