If I'm not mistaken, you can assign the values to the checkboxes with labels. Now the labels is what you will see on the page, however the values you will not. By clicking the checkbox, you should be able to get the value.

So for example you can use -values to index your checkboxes and lables to display 'a.txt' content.

Here is a sample:

print $query->checkbox_group(-name=>'name',
                             -values=>\@metrics_index,
                             -linebreak=>'true',
                             -labels=>\%metrics,
                             -columns=>1);
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Hope this helps and that's what you looking for