In Tk I would use the 'grid' geometry manager, like the following example.

I don't have Prima on my machine, but I noticed in CPAN that there is a Prima::Grid. Perhaps it is similar.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11163718
use warnings;
use Tk;

my $mw = MainWindow->new();
$mw->configure(-title=> "Configuration");

my ($refreshRate, $spectralRes, $showSpectrumHz);
my @tableConfig = ( ['Refresh rate:','60',\$refreshRate,'FPS'],
                    ["Spectral resolution",'10',\$spectralRes,'Hz'],
                    ["Show spectrum up to",'5000',\$showSpectrumHz,'Hz
+']);
$mw->Button(-text => 'Done', -command => sub{ $mw->destroy },
  )->pack(-side => 'bottom', -fill => 'x');
my $table = $mw->Frame->pack;
my $row = 1;
for ( @tableConfig )
  {
  my ($label, $value, $variable, $units) = @$_;
  $table->Label(-text => $label,
    )->grid(-row => $row, -column => 1, -sticky => 'e');
  $$variable = $value;
  $table->Entry(-textvariable => $variable, -width => 8,
    -justify => 'right',
    )->grid(-row => $row, -column => 2, -sticky => 'ew');
  $table->Label(-text => $units,
    )->grid(-row => $row, -column => 3, -sticky => 'w');
  $row++;
  }

MainLoop();

use Text::ASCIITable;
my $t = Text::ASCIITable->new({ headingText => 'Configuration' });
$t->setCols(qw(Item Value Units));
$t->addRow( [ map [ $_->[0], ${$_->[2]}, $_->[3] ], @tableConfig ] );
print $t;
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