Use perl runtime options. Specify input file on command line. Redirect output to a file with your shell.

#!perl -p
use strict;
use warnings;
my  @fields = split /,/;
$fields[3] = $fields[3] > 310 ? 'volmed' :
             $fields[3] == 70 ? 'volmedl':
             $fields[3]
             ;
$_ = join ',', @fields;
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