Is there a reason you are using the /(??{ })/ code block other than just to make a more general solution? This seems to work just fine.

#!/usr/bin/perl

use strict;  
use warnings;

my $tag  = '(?:<[^>]*>)';

while (<DATA>) {
    s/($tag*tacos)/yummy $1/g;
    s/($tag*salad)/green $1/g;
    print;
}

__DATA__
I like tacos.
<5b>I <5c>like <5d>tacos.
I like a salad.
<foo>I <foo>like <foo>a <bar><baz><foo>salad.
<foo>I <foo>like <foo>a <bar><baz><foo>salad <bup>with <bobob>tacos.


I like yummy tacos.
<5b>I <5c>like yummy <5d>tacos.
I like a green salad.
<foo>I <foo>like <foo>a green <bar><baz><foo>salad.
<foo>I <foo>like <foo>a green <bar><baz><foo>salad <bup>with yummy <bo
+bob>tacos.
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