use strict;
use warnings;
use 5.010;

my $line = do { local $/ = undef; <DATA>; };
my @tt_content = ($line =~ m|<\s*tt\s*>([^<]+)</\s*tt\s*>|gim);

say "Found: ";
say for @tt_content;

__DATA__
<Tt>NEVER USE REGEX AS HTML PARSER!</tT>

<
        tt         
                                                                        >"




Hia ha ho "</          tt


>

               <       TT           >a</ tt >