I also don't see a way to do that via configuration options at the moment. But @ARGV is just an array of strings, so you could just do e.g. "s/^-ot\K$/=/ for @ARGV;" before the GetOptions call:

use warnings;
use strict;
use Getopt::Long qw/ GetOptionsFromArray /;
use Test::More tests => 6;

my @argv1 = qw/ -ot=123 arg1 arg2 /;
s/^-ot\K$/=/ for @argv1;
is_deeply \@argv1, ["-ot=123", "arg1", "arg2"];
GetOptionsFromArray(\@argv1, \my %opts1, "ot:s") or die;
is_deeply \%opts1, { ot => 123 };
is_deeply \@argv1, ["arg1", "arg2"];

my @argv2 = qw/ -ot arg1 arg2 /;
s/^-ot\K$/=/ for @argv2;
is_deeply \@argv2, ["-ot=", "arg1", "arg2"];
GetOptionsFromArray(\@argv2, \my %opts2, "ot:s") or die;
is_deeply \%opts2, { ot => "" };
is_deeply \@argv2, ["arg1", "arg2"];
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Update: Switched from Data::Dump to Test::More.