... should be read from text file ...

What text file where? :) How do I post a question effectively?

my @menu = (
    { -label => 'File',
        -submenu => [
            { -label => 'Exit     ^Q',  -value => '\&exit_dialog' },
            { -label => 'Wait     ^W',  -value => '\&remain_dialog'  }
+,
        ],
    },
);
use JSON;
print encode_json({menu=>\@menu});
__END__
{
   "menu" : [
      {
         "-label" : "File",
         "-submenu" : [
            {
               "-label" : "Exit     ^Q",
               "-value" : "\\&exit_dialog"
            },
            {
               "-label" : "Wait     ^W",
               "-value" : "\\&remain_dialog"
            }
         ]
      }
   ]
}
[download]

Then

my $menu = decode_json( path( $jsonfile )->slurp_raw );
my $funcname = $menu->{menu}[0]{"-submenu"}[0]{"-value"};
my $funcref = $callback{$funcname};
$menu->{menu}[0]{"-submenu"}[0]{"-value"} = $funcref;
[download]

I need to read each -label from a line of the text file and pass the name of the menu (== the filename) and the label to the subroutine called when the item is selected.

Your callback will get the menuitem object as argument, you can (usually) query/interrogate/examine this to retrieve the label