G'day Kallewirsch,

It would have been better had you provided all information up-front. From your responses to hippo and haukex, I've determined the following.

You're using WWW::Telegram::BotAPI. This is a front-end to "Telegram Bot API". It's sendMessage method documentation describes "HTML style". You should read that entire section; this extract highlights the main point that applies to you:

"... All <, > and & symbols ... must be replaced with the corresponding HTML entities ..."

From that, and based on what you've revealed so far, you need to modify $av_tmp_LINE before combining it with $av_tmp_STRING. Here's an example:

$ perl -e '
    use 5.010;
    use strict;
    use warnings;

    my $av_tmp_LINE = "Jun  3 23:20:05 f42252s5 postfix/pickup[204714]
+: E1E63A045C: uid=33 from=<www-data>";
    say "BEFORE: $av_tmp_LINE";
    $av_tmp_LINE =~ s/([&<>])/char_to_entity($1)/eg;
    say "AFTER:  $av_tmp_LINE";
    my $av_tmp_STRING = "Logfile: " . "<strong>" . q{$av_obj_TMP->{inp
+ut}} . "</strong>" . " " . $av_tmp_LINE;
    say "\$av_tmp_STRING[$av_tmp_STRING]";

    sub char_to_entity {
        my ($char) = @_;
        state $entity_for = {qw{& &amp; < &lt; > &gt;}};
        return $entity_for->{$char};
    }
'
BEFORE: Jun  3 23:20:05 f42252s5 postfix/pickup[204714]: E1E63A045C: u
+id=33 from=<www-data>
AFTER:  Jun  3 23:20:05 f42252s5 postfix/pickup[204714]: E1E63A045C: u
+id=33 from=&lt;www-data&gt;
$av_tmp_STRING[Logfile: <strong>$av_obj_TMP->{input}</strong> Jun  3 2
+3:20:05 f42252s5 postfix/pickup[204714]: E1E63A045C: uid=33 from=&lt;
+www-data&gt;]
[download]

Note: I haven't tried to interpolate $av_obj_TMP->{input} as I've no idea what its value is.