pp calls scandeps, which in turn uses system when it executes the perl script or code. See the source code here.

This means that the issue can be replicated without pp or scandeps.

perl -e "system ('perl', '-e', 'while (1) {my $x = 1}'); while (1) {my
+ $y = 1}"
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In task manager, kill the parent perl process and the child will keep running.

The reasons for this are best explained by people who know the internals.