for makes $_ an alias, given doesn't:

$ perl5120 -wE'my $x=1; given ($x) { $_ = 2 } say $x'
1
$ perl5120 -wE'my $x=1; for ($x) { $_ = 2 } say $x'
2
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Edit: For some reason I wrote when rather than given above. Now fixed.

Dave.