Update: Does "***Begin $subject" in the equality test need to be "***Begin $subject\n"?

So much to do here.

1. #!/usr/bin/perl -w Warnings on.
2. use strict; aid in debugging.
3. use CGI; You'll get toasted for parsing $QUERY_STRING
4. $| = 1; Let through whatever gets printed right away.

&dienice does nothing that use CGI::Carp qw(fatalsToBrowser); can't do better.

Consider reading the file like this:

my $cgi = new CGI;
my $subject = $cgi->param('subject');

my @b;
{
    local $/="***Begin Here***\n";
    open INF, "< /full/path/to/postall.txt" or die $!;
    @b = grep {
             chomp;    # remove "***Begin Here***\n", then
             # remove marker and return true to grep if match
             s/^\*\*\*Begin \Q$subject\E//;
    } <INF>;
    close INF or die $!;
}
# lines go by
for (@b) {
    s/\n/<br\/>\n/g;
}
print @b;
[download]

Update2: Added chomp, positional ^, and comments to grep code, a litle text cleanup.

After Compline,
Zaxo