Parsing the string is the right approach here IMO. I would split up the parsing into two steps. The first steps identifies "plain text" and "text where angle brackets should be escaped", and the second step then escapes the angle brackets where necessary:

#perl
use strict;
use warnings;

sub escape {
    my( $str ) = @_;
    $str =~ s!<!&lt;!g;
    $str
}

while( <DATA>) {
    s!(?:^|\G)([^`\\]* # plain string without escape or backtick
               |\\.    # anything, escaped
               )*
               (`      # backtick
                 (?:[^\\`]*|\\.) # only escaped backticks
                `)     # closing backtick
     !$1.escape($2)!sgex;
    print
};

__DATA__
This is `my <string>` that I want to modify because the angle bracket 
+is between backticks
This is `my &lt;string>` that I want to modify because the angle brack
+et is between backticks
This is `a <string> I don't want to modify because it's not between ba
+ckticks
This is \`another <left_angle_bracket>`I don't want to modify because 
+the first backtick is escaped but I do want to modify <the_last_left_
+angle_bracket>` between the backticks

[download]

At least for these test cases, this works, but I'm not sure if there is/should be a way to quote angle brackets too, like \<. This would mean you have to introduce < into the appropriate character classes as well.

Update: Please see also tybalt89's reply, which contains some good optimizations that prevent backtracking and do the "no/escaped backtick" matching in a more modular way.

In reply to Re: Replacing left angle bracket with HTML entity when between two backtick characters by Corion
in thread Replacing left angle bracket with HTML entity when between two backtick characters by nysus

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