my $last_pos = 0;               # start at the beginning
# later....
seek(FILE, $last_pos);          # go to where I was
while (<FILE>) {                # start reading line at a time
     if (/$userid/) {           # sure about the /o, BTW?
        $last_pos = tell(FILE); # remember this spot
        ....                    # other stuff

##</code><code>##

my %index;
while (<FILE>) {
    $userid = split(...)     # It's in there somewhere, right?
    $index{$userid} = tell(FILE};
}
# later...
seek(FILE, $index{$userid});
$_ = <FILE>;
($inv, $date, $amt) = split (...);