Using the modulo (%) operator is probably a bit more idiomatic way to do this rather than explicitly resetting the counter.

my $count = 0;
while( $count < 30 ) { print "count $count\n" if ++$count % 3 == 0; }
[download]

Update: And a gripe for the OP: your open statement would be better written:

my $outfile = "$rootpath$categoryDir$frontPage";
open(TXTFILE, ">", $outfile )
  or die "Can't open output file '$outfile': $!\n";
[download]

You'll be thanking yourself for providing a useful error message n months down the road when something changes and you're one step closer to fixing it rather than trying to chase down a mysterious "write on a closed filehandle" error.