G'day redapplesonly,

"I am working on a Perl (v5.30.0) script that needs to use stateful information from the previous run of the script."

All of your steps, and other design notes, seem spot-on to me. :-)

"But here's the problem: What about the first time the script runs?"

That's a very valid point. I don't use Storable very often, but when I do I encounter the same issue. I usually handle this by dealing with it before runtime execution starts in an INIT block.

Here's a rough example of how I might have written your code (pm_11148149_storable_first_run_0.pl):

#!/usr/bin/env perl

use 5.030;
use warnings;

use Storable;

my ($hashfile, $serial_data_for);
INIT {
    $hashfile = 'pm_11148149_storable_first_run.sto';
    $serial_data_for = -e $hashfile ? retrieve($hashfile) : {};
}

my @keys = qw{key1 key2 key3};
my @vals = qw{data1 data2 data3};
$serial_data_for->@{@keys} = @vals;

store($serial_data_for, $hashfile);
[download]

I added some extra statements for reporting/demo purposes (pm_11148149_storable_first_run_1.pl):

#!/usr/bin/env perl

use 5.030;
use warnings;

use Storable;
use Data::Dump;

warn "Perl version: $^V\n";

my ($hashfile, $serial_data_for);
INIT {
    $hashfile = 'pm_11148149_storable_first_run.sto';
    $serial_data_for = -e $hashfile ? retrieve($hashfile) : {};
}

say 'Serialised data:';
dd $serial_data_for;

my @keys = qw{key1 key2 key3};
my @vals = qw{data1 data2 data3};
$serial_data_for->@{@keys} = @vals;

store($serial_data_for, $hashfile);
[download]

Here's some equivalent output to your sample run:

ken@titan ~/tmp
$ rm pm_11148149_storable_first_run.sto

ken@titan ~/tmp
$ ls -l pm_11148149_storable_first_run.sto
ls: cannot access 'pm_11148149_storable_first_run.sto': No such file o
+r directory

ken@titan ~/tmp
$ ./pm_11148149_storable_first_run_1.pl
Perl version: v5.30.0
Serialised data:
{}

ken@titan ~/tmp
$ ls -l pm_11148149_storable_first_run.sto
-rw-r--r-- 1 ken None 69 Nov 12 13:00 pm_11148149_storable_first_run.s
+to

ken@titan ~/tmp
$ ./pm_11148149_storable_first_run_1.pl
Perl version: v5.30.0
Serialised data:
{ key1 => "data1", key2 => "data2", key3 => "data3" }

ken@titan ~/tmp
$
[download]

Notes:

• I concur with ++choroba's suggestion to use the builtin exists() function directly, rather than rolling your own DoIHaveThisData(). While abstracting functions into subroutines for reuse is often a very good idea, I don't see any benefit in this instance. If you did consider it to be necessary, I'd keep it very simple along the lines of:

  sub DoIHaveThisData {
      my ($hash, $key) = @_;
      return exists $hash->{$key};
  }
  [download]

• You may need to look at "perlref: Postfix Reference Slicing", and perhaps even "perldata: Slices", if you're unfamiliar with how I've populated the hashref.
• A .sto extension, to identify Storable files, is widely used. It's not a requirement; it might qualify as a convention.

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