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my @keys_to_process = $x eq 'key_1'       ? keys %my_hash
                    : exists $my_hash{$x} ? ($x)
...
foreach $key (@keys_to_process) {
    do_something_with($key);
    }

my @keys_to_process = ! exists $my_hash{$x} ? ()
                    : $x eq 'key_1'         ? keys %my_hash
                    :                         ($x)
                    ;