This is do the trick for arbitrary comment flags:
/\Q$start\E(.*?)\Q$end\E/sg

I wrote a small test program:

#!/usr/bin/perl

#get start and end comment
my $start = $ARGV[0];
my $end   = $ARGV[1] || "\n";

open(FILE, "test.txt") || die;
{
    local $/ = undef; #set to 'slurp' mode
    $_ = <FILE>;      #read entire file into $_
}
close FILE;

#
#print all comments that are matched in file
#
while( /\Q$start\E(.*?)\Q$end\E/sg )
{
    print $&, "\n";
}
[download]

For a test.txt I used:

blah blah blah blah blah 
*** multi
    line
    comment **!!
blah blah blah blah blah blah blah blah blah blah 
blah *** inline comment **!! blah blah

/* c comment 1 */ 
blah blah blah blah blah blah blah blah blah blah 
blah blah blah blah blah blah blah blah blah blah 
/* 
 * c comment 2 
 */
 
blah blah // c++ comment 1
blah blah // c++ comment 2
[download]

For my execution results I got (my exe is called regex.pl):

prompt$ regex.pl '***' '**!!'
*** multi
    line
    comment **!!
*** inline comment **!!
prompt$ regex.pl '/*' '*/'
/* c comment 1 */
/*
 * c comment 2
 */
prompt$ regex.pl '//'
// c++ comment 1
 
// c++ comment 2
[download]