It's (part) of a list slice. List slices are documented in perldata.

[ Of the three existing posts, neither of those facts were mentioned!?! ]

A list slice has the following form:

( EXPR )[ EXPR ]
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Of the scalars returned by the left EXPR, the slice returns those identified by the indexes returned by the right EXPR.

( 'foo', 'bar', 'baz' )[ 0, 2, 4 ]   # 'foo', 'baz', undef
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The scalars are returned in the order identified by the indexes. Negative indexes index from the end like with arrays. (-1 = last, -2 = second last, etc)