use File::Find;
use File::Glob ':glob';

use strict;

find(\&wanted, "c:/perl58/bin"); #replace with your directory

sub wanted {
    if ((-d $File::Find::name) && ($_ ne ".") && ($_ ne "..")) {
        my @shtml_files = bsd_glob("*.shtml");
        foreach my $shtml_file (@shtml_files) {
            print $shtml_file;
            open(SHTMLFILE, "<", $shtml_file);
            my $buffer;
            read(SHTMLFILE, $buffer, 10000); #give some big number, which exceeds the size of all your .shtml files   
            close(SHTMLFILE); 
            $buffer =~ s/<a href="main\.php\?page=(.*?)\"/<a href="main.php?id=$1"/g;
            open(SHTMLFILE, ">", $shtml_file); 
            close(SHTMLFILE); 
        }
    }       
}