You've been pointed to nodes that detail that homework questions are not liked here. I found it a neat challenge to write a program that does what you want but is a bit opaque in what it does:

#!/opt/perl/bin/perl -w
no strict;
no warnings;

# Handle data by prefixing
# the string with "#". The string
# is passed in implicitly via $_
sub handle_data { do{eval}->() };

$/ = "";
while (<DATA>) {
    handle_data;
};

__DATA__
# This program was written by Corion
for   # Anonymous Monk - http://www.perlmonks.org
( # see http://perlmonks.org/?node_id=567615
sub { $/ = "\n" }, 
sub { *DATA = *ARGV; $(}) 
    {$_->()};
  sub { *::handle_data = sub { substr $_, $(,$), "#"; print }
};
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