Perhaps the following, which uses a negative lookhead and a negative lookbehind, will be helpful:

use strict;
use warnings;

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

sub rep {
    $_[0] or return $_[0];
    $_[0] =~ s/<(?!url)|(?<!url)>//g;
    return $_[0];
}
[download]

Output:

this is not OK  but this is ok <url>
[download]

Your sub returns '' if $_[0] evaluates to false. This means that 0 and !defined $_[0] would convert to '', and perhaps that's your intent. In these cases, the above just returns what was sent (if anything), so you may need to adjust it.