If you just want to match $PrevMsgLink as a substring, do:

my $topic_obj = $mech->find_link (text_regex => qr/\Q$PrevMsgLink\E/i 
+);
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The \Q .. \E construct backlash-escapes all special characters (it's a shortcut to the quotemeta function). That way, whatever $PrevMsgLink contains, it will match it as a literal string.