It can be done as :

my $file_size=-s "folder/Excel_12345.xls" ;
	print $q->header(
		-type=>"application/vnd.ms-excel",     #MIME type
		-attachment=> "folder/Excel_12345.xls",
		-Content_Length=>"$file_size"          #File size
		);
		open (INFILE,"folder/Excel_12345.xls" ) or die("FAILED TO OPEN Excel FILE"); 
		binmode(INFILE);
		while (<INFILE>){
			print $_;
		}
		close(INFILE);          
		unlink ("folder/Excel_12345.xls" ); #delete file from server
		exit;

It’s important that it should be written before printing the header. ie., print $q->header();