my $includepath="somefolder/somefolder/";
$html =~ s/<!--#include file="([^"]+)"-->/&add_template($includepath.$1)/gexi;
print $html;

sub add_template{
        my( $filename ) = @_;
        if(!-f $filename){
             return "Could not include template '$filename' because: NO  FILE"
        }elsif(!-r $1){
             return "Could not include template '$filename' because: NO ACCESS TO FILE";
        }else         {
         return read_file($filename);
        }
}