# Load old list.
my @sets = do {
   open(my $fh, '<', $file)
      or die("Unable to open set list \"$file\": $!\n");

   local $/ = '<br>';

   <$fh>
};

# Add new item to the top.
unshift @sets, "a new set - line 7<br>";

# Keep the top 5.
if (@sets > 5) {
   @sets = @sets[0..4];
}

# Save new list.
{
   open(my $fh, '>', $file)
      or die("Unable to create set list \"$file\": $!\n");

   print $fh @sets;
}