It looks like you can't do that with '#' in a format. Sigh.

So (There Is More Than One Way ... after all), we use sprintf() and pre-format our field.

my $hh1_char = sprintf("%02d", $hh1);
my $mm1_char = sprintf("%02d", $mm1);
my $ss1_char = sprintf("%02d", $ss1);
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Then use $hh1_char, etc, in your format list. (Changing the appropriate '#'s to '<'s, of course.)

The sprintf() format ("%02d") says we are going to have decimal digits, two of them, and fill any leading spaces with zeros please.