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

my %index;
while (<FILE>) {
...
seek(FILE, $index{$userid});
$_ = <FILE>;
($inv, $date, $amt) = split (...);