G'day Statue,

Welcome to the monastery.

That's illegal; it's got nothing to do with the array:

$ perl -E '
    my $x = "z";
    my $y = 1;
    my $$x = $y;
    say $z;
'
Can't declare scalar dereference in "my" at -e line 4, near "$x ="
Execution of -e aborted due to compilation errors.
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If you didn't want a my variable, you could do this:

$ perl -E '           
    my $x = "z";
    my $y = 1;
    $$x = $y;
    say $z;
'
1
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You could eval a string containing that code but the scope of the lexical variable so declared would only be that string:

$ perl -E '
    my $x = "z";
    my $y = 1;
    eval "my \$$x = \$y; say \">>>\$z<<<\"";
    say ">>>$z<<<";
'
>>>1<<<
>>><<<
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If you're actually trying to write code that writes code, your best bet might be to output the generated code to a file (e.g. script_my_script_wrote.pl) which you could subsequently run in the normal fashion. For example, this code in your generating script:

print $out_script_fh "my \$$splitter[0] = $splitter[1];";
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Would write this line into script_my_script_wrote.pl (or whatever you call it):

my $testval = 1;
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[Aside: this is your first post so the title "Self-writing code revisited" doesn't make much sense. Perhaps a link to whatever you're revisiting would help to clarify what you mean.]