use strict;
use warnings;

my $string;
my $startOfIframe;
my $endOfIframe;

open TEST, "<test_string.txt"  or die "Cannot open:$!";
$string = readline TEST;
close TEST;

$startOfIframe = index $string, '<iframe' ;
$endOfIframe   = index $string, '</iframe>';

while
(	$startOfIframe != -1
)
{
	print substr ( $string, $startOfIframe, $endOfIframe - $startOfIframe ) . '</iframe>'. "\n";

	$startOfIframe = index $string, '<iframe', $endOfIframe;
	$endOfIframe   = index $string, '</iframe>', $startOfIframe;
}