You really need to post the code within <code> tags as described in Markup in the Monastery at the bottom of the submit-page.

If you suspect that someting in sub() might be adversely affecting repeated calls to getgrent(), consider defining an array and pushing all matching names onto it.   Then, after the getgrent loop has finished, use a second loop to process the entries in the array.   Similar side-effects can happen e.g. when processing files within a loop that is scanning directories, on some types of machines.   So, first do one, then the other.