Would this not be a case where File::Find would be appropriate to find those files you need? Perhaps something along the lines of:

#
# UNTESTED CODE
#
use File::Find;

# If desired, change following to absolute path(s) to search
my @directories = ('.');
find(\&search_and_replace, @directories);

sub search_and_replace {
  # Item from search is a file
  -f && 
  # Item's full name ends with '.shtml'
  $File::Find::name =~ m/\.shtml$/ &&
  # Rename file so there is a backup
  rename($File::Find::name, $File::Find::name . '.bak');
  # Read from original (now .bak), writing to target
  open(INF, $File::Find::name . '.bak') 
      or die('Input: ', $!, "\n");
  open(OUTF, '>' . $File::Find::name) 
      or die('Output: ', $!, "\n");
  {
    # Localize $_ to prevent potential problems with call
    local($_);
    # Loop thru file, doing replacements
    while ($line = <INF>) {
      $line =~ s!(href="?)main\.page\?page=!$1/id=!g;
      print(OUTF $line);
    }
  }
  close(OUTF);
  close(INF);
}
[download]

Hope that the idea above at least helps.

Update: Added comments for clarification.