...
my $test = "this is not OK < but this is ok <url>";
my $res = rep( $test );
print "\n\n$res\n\n";

sub rep {

  if ( $_[0] ) {
      
    $_[0] =~ s/<|>//g unless $_[0] =~ /(\<\burl\b\>)/i;
    
    return $_[0];
    
  }else {
    return '';
  }

}
...