# $userid already populated

# first check cache
open(CACHE, "<cache.txt") or die "Oops $!";
my @cached = <CACHE>;
close CACHE;

# now get the most recently cached line
# as a line could be cached multiple times
my @lines = grep{/$userid/}@cached;
my $line  = pop @lines || '';

# only look though the file if we have not cached the 
# data associated with $userid recently
if (not $line) {
    open(FILE,"FILE.txt") or die "Oops $!";
    while(<FILE>) { 
        if(/$userid/o) {
            $line = $_;
            close FILE;
            last; # exit as soon as we find $userid;
        }   
    }
    close FILE;
}

# do the stuff
if ($line) {
    chomp $line;
    cache($line);
    my ($inv, $date, $amt)= split(/[|]/);
    #code to build hash.
}
else {
    warn "Can't find userid: $userid\n";
}

sub cache {
    my $line = shift;
    open(CACHE, ">>cache.txt") or die "Oops unable to open cache to append: $!";
    flock CACHE, LOCK_EX;
    print CACHE "$line\n";
    close CACHE;
}


# to limit the cache size you will need to clean it up
# from time to time say via a cron tab. this sub take 
# the desired maximum cache size as its argument
sub clean_cache {
    my $max_length = shift;
    open(CACHE, "+<cache.txt") or die "Oops unable to open cache to R/W: $!";
    flock CACHE, LOCK_EX;
    my @cached = <CACHE>;
    my $start = $#cache - $max_length;
    $start = 0 if $start < 0;
    seek CACHE, 0, 0;
    truncate CACHE, 0;
    print CACHE @cache[$start..$#cache];
    close CACHE;
}