Update: Not directly, by any means I've found (nor by any explicitly sanctioned by </update> shmem found a "direct" means... but see Friedl, Mastering Regular Expressions, anyway.

However: TIMTOWTDI:

#!/usr/bin/perl
use strict;
use warnings;

# 829735

my $replacement ="*"x80;

print "$replacement \n";

my @logfile = <DATA>;
for my $logfileline(@logfile) {
    $logfileline =~ s|^\-+|$replacement|;
    print "$logfileline \n";
}

__DATA__
phoney content
--------------------------------------
more dummy stuff
[download]

which appears to do what you want:

ww@GIG:~/pl_test$ perl  829735.pl
**********************************************************************
+********** 
phoney content
 
**********************************************************************
+**********
 
more dummy stuff 
ww@GIG:~/pl_test$
[download]

NB: \-+ in the regex because I'm too lazy to count the hyphens. :-)