Not quite true. I will not recommend the use of

[ $a => $b ]->[ $b <= $a ]
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as it relies on too much syntax magic.

The first set of brackets [ ] is used to indicate an anonymous array, whereas the second set of brackets ->[ ] de-references the array.

=> is used as a fat comma (not greater-than-or-equal-to), whereas <= is used as less-than-or-equal-to.

Hence,

[ $a => $b ]->[ $b <= $a ]

is same as

[ $a, $b ]->[ ($b <= $a) ]

which either evaluates to

[ $a, $b ]->[0]  # returning $a
or
[ $a, $b ]->[1]  # returning $b
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