You can't stop an exit, but you can catch the return code using my $ret = system("$0 arguments") or something like it.

The usual approach is to use die() instead:

sub some_sub {
   die "Error" if ($some_condition);
}

eval { 
  my $value = some_sub();
  # do stuff with $value
};
if (my $exception = $@) {
  # exception!
  handle error here
}
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