my $max_line_count = $whatever;
my $file_count = my $line_count = 0;
my $outfh;

sub printAndCount
{
    local $_ = shift;
    my $lines2print = tr/\n/\n/;  # simple way to count \n's
    if ( $file_count == 0 or 
         $line_count + $lines2print > $max_line_count )
    {
        close $outfh if ( $file_count );
        open $outfh, sprintf( ">track%03d.txt", ++$file_count ) or die "open failed: $!";
    }
    print $outfh;
    $line_count += $lines2print;
}