Further, the toggle button does not toggle between normal and disabled, so apparently $test1 -> Entry is not the "right" Entry... ?

Fixed your code:

use strict;
use warnings;
use Tk;
my $test1;
my $var = "55";
my $mw = MainWindow->new();
my $FrameOpt = $mw -> Frame;
my $exit_b = $mw->Button(-text => 'Exit', 
  -command => sub { exit })->pack; 
  
$mw->Button(-text => "Toggle Textbox", 
           -command => sub { 
             my ($entry) = grep { $_->class eq "Entry" } $FrameOpt->ch
+ildren;
             $entry-> configure(-state => $entry->cget('-state') eq "d
+isabled" ? 'normal' : 'disabled'); 
  })->pack; 

sub GuiTextEntryLabelOnLeftCreate {
    my ($frame, %x_args) = @_;  
    my $label;  
    my $text = $x_args{'-text'};
    delete $x_args{'-text'};
    
    $label = $frame 
        -> Label (
               -text => $text,
               -foreground => "black"
           )

        -> grid (
               $frame -> Entry (%x_args) ,
               -sticky => 'w' ,
               -pady   => 2 
           );

    $label;
}

$test1 = GuiTextEntryLabelOnLeftCreate
(
   $FrameOpt ,
   -text      => "Test value"   ,
   -textvariable => \$var ,
   -width     => 6,
   -state => "disabled",
   -foreground => "gray",
);

$FrameOpt->pack;

MainLoop();
[download]

Your problem was that $test1 did not have any "Entry" .. the entry is a child of $FrameOpt.

                "These opinions are my own, though for a small fee they be yours too."

Thanks for your fix! FYI, this is a 26kLOC perl/Tk GUI, and while I've reviewed potential children of $FrameOpt there's a possibility in the future that there could be a match to an unintended target.

Is there a way to narrow down the search, or is the $test1 handle useless? For instance, I tried $test1 -> $FrameOpt -> Entry but got an error "Can't locate auto/Tk/Frame=HASH(0x68d3998).al" so that's obviously not the approach.

It could be there is no solution without a different subroutine call, and I don't mean for you to have to delve into that :-) In which case I would carefully comment.