$_++ for map $_, @a;
generates an anonymous list with map, then iterates through it with for incrementing all its values. Once for is done, the list is thrown away (expensive no-op).
$_++ for my @b = map $_, @a;
generates a list with map and throws it in @b, then iterates through it with for incrementing all values as before. This time, the list is not anonymous and may be accessed by using @b and it's not thrown away. Once map is evaluated, values in @b are totaly independent from values in @a.
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