Well, to remove the everything before the fifth pound...

Code:
$string =~ s/^(\#.*){4}\#/\#/;

# or

$string =~ s/^(#.*){4}#/#/;
Like I said, I don't think you need to escape the pounds, but it shouldn't break anything if you do.

Now this will modify the string, so you will loose everything at the beginning of the string. There are way around that. You could copy the string and work the with copy.

Another idea is....

Code:
my $string;    #the pound enriched string
my @subStrings;   #each element is a peice of $string without the #'s
my $result;    #the resulting string
my $targetPound;  #this is the index of the #s in the string that you want to start reading.  0 is the first

@subStrings = split /#/, $string;
$result = join /#/, $subStrings[$targetPound .. -1]