Fair enough. For what it is worth: I am running Prima on Strawberry Perl (the 5.38 PDL edition) but don't recall its history. Sometimes Prima tests act up, (in particular those running tests on character display. Perhaps I skipped tests.

According the CPAN Testers I'm more likely to have success with 1.74 (penultimate version) than 1.75 (current version). Actually, if I use force I can install 1.74 against Strawberry 5.22 at work. Does this do what you want:

use Prima qw(Application Label InputLine);

my $main = Prima::MainWindow->create(
    text => 'Configuration',
    backColor => cl::White,
    menuItems =>
    [   ['~File' => [['~Quit' => 'Ctrl+Q', '^Q',
                   sub { $::application->close } ], ],
     ],
    ]
);

my $fps = $main->insert(
    Widget =>
    pack => { side => 'top' },
);
$fps->insert(
    Label =>
    text => 'Refresh rate: ',
    width => 200,
    pack => { side => 'left' },
);
$fps->insert(
    InputLine =>
    text => '60',
    width => 100,
    pack => { side => 'left' },
);
$fps->insert(
    Label =>
    text => 'FPS',
    width => 50,
    pack => { side => 'left' },
);

my $res = $main->insert(
    Widget =>
    pack => { side => 'top' },
);
$res->insert(
    Label =>
    text => "Spectral resolution",
    width => 200,
    pack => { side => 'left' },
);
$res->insert(
    InputLine =>
    text => '10',
    width => 100,
    pack => { side => 'left' },
);
$res->insert(
    Label =>
    text => 'Hz',
    width => 50,
    pack => { side => 'left' },
);

my $limit = $main->insert(
    Widget =>
    pack => { side => 'top' },
);
$limit->insert(
    Label =>
    text => "Show spectrum up to",
    width => 200,
    pack => { side => 'left' },
);
$limit->insert(
    InputLine =>
    text => '5000',
    width => 100,
    pack => { side => 'left' },
);
$limit->insert(
    Label =>
    text => 'Hz',
    width => 50,
    pack => { side => 'left' },
);

Prima->run;
[download]

Update: Sorry, forgot about the "no counting pixels" criteria.

Optimising for fewest key strokes only makes sense transmitting to Pluto or beyond