If you have a token like this, then regex is not the right tool. Use tr to count < and > and just see if they are equal in number. 
#!/usr/bin/perl -w
use strict;

for (
    '<1>', # want match
    '<<2>>', # want match
    '<<3>', # want fail
    '<<<4>>', # want fail
    '<<5>>>>', # want fail
    )
{
   my $left = tr/<//;
   my $right = tr/>//;
   if ($left == $right)
   {
      print "$_  match\n";
   }
   else
   {
      print "$_ fail\n";
   }
}
__END__
Prints:
<1>  match
<<2>>  match
<<3> fail
<<<4>> fail
<<5>>>> fail

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In reply to Re: experimental regex (?(?{code}) conditional (??{ 'code' }) ) by Marshall
in thread experimental regex (?(?{code}) conditional (??{ 'code' }) ) by Anonymous Monk

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