I would imagine that Abigail avoided:

system("mkdir -p $foo");
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because that would be interpreted by the shell, and would foul up if (for example) $foo contained a filename with spaces. The advantage of using the arrow instead of the comma is that you can get away with less quoting, compare:

system mkdir => -p => $foo;
system 'mkdir', '-p', $foo;
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Same number of chars, arguably more readable.