my $pos = 0;
   while ( $pos < length( $all ) )
   {    my $si = index ( $all, "<START>", $pos );
        if ( $si < 0 ) { last; }
        my $sj = index ( $all, "<END>", $si+1 );
        if ( $sj > $si+1 )
        {    ... Collect you string with 'substr' etc.
        }
        $pos = $sj + 4;     # set search for finding next text.
   }