use strict;
my $input='
This is sample <This is sample text>
This is sample &lt;Th<a> </a>is is  sa<not a>mple text <a>this is sample text</a> sample text >';

my @arr=$input=~m#&lt;[^<>]+(?:<a>.+?</a>[^<>]*)?>#g;
print join "\n",@arr;

##</code><code>##

qr {&lt;(?:[^<>]+|<a>|</a>)*>}

##</code><code>##

qr {&lt;(?:[^<>]+|<a>|</a>)*[^>]*>}

##</code><code>##

my $re;
$re=qr{(?:[^<>]+|<a>(??{$re})?</a>)*};
my @arr=$input=~m#&lt;$re>#g;