You can open both files in Perl, which has no line length limits:

{
    my ( $in, $out );
    open $in, '<', $filepath or die $!;
    open $out, '>', $newpath or die $!;
    while (<$in>) {
        # process $_ which is the current line
        $_ = quux($_);
        print $out; # assumes $_ has everything to print now
    }
    close $out or die $!;
    close $in;
}
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There are lots of ways to do this, many shortcuts. I showed this one because it's explicit about what it does. The use of lexical $in, $out as filehandles is relatively new, as is the three-argument open. For older perl, use globs like \*IN, \*OUT for filehandles, and concatenate the direction arrows '<', '>' with a space and the file path.

Update: After a lot of heat and little light, it seems that what you want is getgrent. If it fails, your /etc/group file is probably broken. See also 'perrldoc User::grent'.

After Compline,
Zaxo