$thiscust = (split /\//, $fields[4])[1];
if ( ! $cust{$thiscust} ) {
    print "customer $thiscust\n";
    $cust{$thiscust} = $thiscust;
    open (my $fh, ">> ${prefix}${thiscust}${suffix}")
        or die ("Cannot open ${prefix}${thiscust}${suffix}: $!");
    $cust{$thiscust} = $fh;
}
my $fh = $cust{$thiscust};
print $fh $_;