For markup use the appropriate parsing module. That will save you a pile of time and substantive questions won't get clouded by the "but you should be using a module" answer - you've preempted that answer and move on to real issues.

So, lets look at the real issue without the distraction of HTML, XML, ... . Consider:

use strict;
use warnings;

my $match1 = 'something ';
my $fill = 'xxxxxx ';
my $match2 = 'somethingelse ';
my $nomatch = 'nomatch ';
my @targets = (
    "$nomatch$fill$match1$match2$nomatch",
    "$nomatch$fill$match1$fill$match2$nomatch",
    "$nomatch$fill$match1$fill$nomatch$fill$match2$nomatch",
    );
    
for my $test (@targets) {
    if ($test =~ /$match1 ((?:(?!$nomatch) .)*) $match2/x) {
        print "Matched >$1< for: $test\n";
    } else {
        print "Failure for $test\n";
    }
}
[download]

Prints:

Matched > < for nomatch xxxxxx something somethingelse nomatch 
Matched > xxxxxx < for nomatch xxxxxx something xxxxxx somethingelse n
+omatch 
Failure for nomatch xxxxxx something xxxxxx nomatch xxxxxx somethingel
+se nomatch
[download]

The trick is the (?:(?!$nomatch) .)* which will only match a character if it is not the start of the nomatch criteria.