#!/usr/bin/perl -lanF,

    # if the current key (in $F[0]) is not the same as the
    # last key we saw, print out the merged output line for
    # the last key and then start a new merged output
    # line for the current key

    if ($last_key ne $F[0]) {
        print $merged if $merged;
        $merged   = $_;
        $last_key = $F[0];
    }

    # otherwise, the current key is the same as the last,
    # and so we can merge this line's value portion (in
    # $F[1]) with the previous

    else {
        $merged .= ",$F[1]";
    }

    # when we reach the end of the file, we must print
    # the final merged output line

    END { print $merged if $merged }