The easiest way to do this is not with CSS but by using the labels for the buttons. Reading the CGI documentation you will find you can create a HASH which ties a label to the value for each button.

radio_group(-name=>'group_name',
            -values=>['eenie','meenie','minie'],
            -default=>'meenie',
            -linebreak=>'true',
            -labels=>\%labels,
            -attributes=>\%attributes);
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Supplying a ref to this hash allows the labels to be totally unrelated to the value and will allow you to achieve what you want.

Even more interesting from the layout perspective, is the ability to have the button array created within an HTML table, viz:

All modern browsers can take advantage of the optional parameters -row
+s, and -columns. These parameters cause radio_group() to return an HT
+ML3 compatible table containing the radio group formatted with the sp
+ecified number of rows and columns. You can provide just the -columns
+ parameter if you wish; radio_group will calculate the correct number
+ of rows for you.
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jdtoronto