I would guess that '+' means the return value of a_method_call will be interpreted as a number. Secondary effect should be that it prevents indirect object invocation of ok, but the writer could as well just have used parenthesis to get this effect without confusing everyone else.

At least thats my take, I might be wrong

Unary-"+" does not numify (or stringify). It's a no-op.

>perl -E"$_ = 'abc'; say +$_;"
abc

>perl -E"$_ = [qw( a b c )]; say @{ +$_ };"
abc
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It doesn't even influence context. Not even void!

>perl -E"sub f { say( ( (wantarray && 'list' ) // 'void' ) || 'scalar'
+ ); } +f(); $_=+f(); ()=+f();"
void
scalar
list
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What you said is the secondary effect is the only effect.

I agree. Given that it isn't some weirder operator like the solitary underscore, it had just better be a unary plus operator and yes, I agree also that brackets are a better idea - sometimes I even think the aura of inconsistency (there is of course a technical distinction) of whether brackets are required by a function or command is a bad thing.

One world, one people

with

$ perl -MO=Deparse,-p  -e " ok +Some::Package::Foo->a_method_call($var
+), 'test succeeded'; "
(('ok' + 'Some::Package::Foo'->a_method_call($var)), '???');
-e syntax OK

$ perl -MO=Deparse,-p  -MTest::More -e " ok +Some::Package::Foo->a_met
+hod_call($var), 'test succeeded'; "
use Test::More;
ok('Some::Package::Foo'->a_method_call($var), 'test succeeded');
-e syntax OK
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without

$ perl -MO=Deparse,-p  -e " ok Some::Package::Foo->a_method_call($var)
+, 'test succeeded'; "
('Some::Package::Foo'->ok->a_method_call($var), '???');
-e syntax OK

$ perl -MO=Deparse,-p  -MTest::More -e " ok Some::Package::Foo->a_meth
+od_call($var), 'test succeeded'; "
use Test::More;
ok('Some::Package::Foo'->a_method_call($var), 'test succeeded');
-e syntax OK
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Anonymous brother, can you please point out this difference more clearly?   I am old, and mine eyes are dim.

Ideas

Increase font size?

Increase contrast?

Disable code wrapping (temporarily)?

Switch themes (temporarily)?

Run through perltidy/ppi2html/gvim...pastebin...?

Try this, with

# -e " ok +Some::Package::Foo->a_method_call($var), 'test succeeded'; 
+ "

(('ok' + 'Some::Package::Foo'->a_method_call($var)), '???');


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

(
    (
        'ok'
         + 
        'Some::Package::Foo'
        ->
        a_method_call
        (
            $var
        )
    )
    ,
     '???'
)
;


##################################################################

# -MTest::More -e " ok +Some::Package::Foo->a_method_call($var), 'test
+ succeeded'; "

use Test::More;
ok('Some::Package::Foo'->a_method_call($var), 'test succeeded');


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

use Test::More;
ok
(
    'Some::Package::Foo'
    ->
    a_method_call
    (
        $var
    )
    ,
     'test succeeded'
)
;


##################################################################
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without

# -e " ok Some::Package::Foo->a_method_call($var), 'test succeeded'; "

('Some::Package::Foo'->ok->a_method_call($var), '???');


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

(
    'Some::Package::Foo'
    ->
    ok
    ->
    a_method_call
    (
        $var
    )
    ,
     '???'
)
;


##################################################################

# -MTest::More -e " ok Some::Package::Foo->a_method_call($var), 'test 
+succeeded'; "

use Test::More;
ok('Some::Package::Foo'->a_method_call($var), 'test succeeded');


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

use Test::More;
ok
(
    'Some::Package::Foo'
    ->
    a_method_call
    (
        $var
    )
    ,
     'test succeeded'
)
;
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