(just now came across my post through a Google search, ... here's the solution for completeness sake)

"SelChange" event: quite right! That's what hooks are for.

$myRichEdit->Hook(EN_SELCHANGE, \&SelChangeEvents_Handler);

sub SelChangeEvents_Handler {
    
    my ($object, $wParam, $lParam, $type, $msgcode) = @_;
    
    return unless($type == WM_NOTIFY);
    return unless($msgcode == EN_SELCHANGE);
    
    my ($hwndFrom, $idFrom, $code, $cpMin, $cpMax, $seltyp) = 
      unpack("LLLLLL", unpack("P24", pack("L", $lParam)));
    
    return unless($hwndFrom == $object->{-handle});
    
    my $haveSelection = ($cpMin != $cpMax);
    
    if($haveSelection){
        print STDERR " we have a selection!";
    }
}
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