Instead of searching for the entry, you could explicitly attach a variable ($entry_1) to it at creation time:

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; 
my $entry_1; 

$mw->Button(-text => "Toggle Textbox", 
           -command => sub { 
             #my ($entry) = grep { $_->class eq "Entry" } $FrameOpt->c
+hildren;
             $entry_1-> configure(-state => $entry_1->cget('-state') e
+q "disabled" ? '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 (
               $entry_1 = $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();
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