for (
    '<1>', # want match
    '<<2>>', # want match
    '<<3>', # want fail
    '<<<4>>', # want fail
    '<<5>>>>', # want fail
    ){
  use vars qw' $LEN ';
  print "$_\n";
  print "  $_\n" for m(
(?<! < ) # no preceeding <
(?><+)(?{ $LEN = length $^N; })
( [^><]* )
(?(?{ $LEN > 0 })(??{ '>' x $LEN }))
(?! > )  # no following >
)x;
print "    LEN = $LEN\n";

}
__END__
<1>

    LEN = 0
<<2>>

    LEN = 0
<<3>

    LEN = 0
<<<4>>

    LEN = 0
<<5>>>>

    LEN = 0