#!/usr/bin/perl

use strict;  
use warnings;

my $tag  = '(?:<[^>]*>)';
sub word {
    my $word = shift;
    qr /$tag*$word/;
}

while (<DATA>) {
    s/((??{ word 'tacos' }))/yummy $1/g;
    s/((??{ word '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 <bobob>tacos.