my %cache;
my ($ncalls, $nhits);
sub first_line
{
    my ($filename) = @_;
    $ncalls++;
    if (exists $cache{$filename}) {
        $nhits++;
    }
    else {
        open( my $fh, $filename ) or die "Cannot open '$filename': $!\n";
        $cache{$filename} = <$fh>;
    }

    return $cache{$filename};
}

END { print "first_line returned cached value $nhits times in $ncalls calls\n" }