${$varname} works by looking in the local package for that variable name. Since you've used my, your variables won't be found there.

To achieve your stated immediate aim here of assigning to the variables you just assigned to (this "zero padding" is itself not a great idea, use the proper timestamp idea instead), you'd do:

$_ = zeropad($_) for $sec, $min, $hour, $day, $month;
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But that itself won't affect the "timestamp" (how could it?). If you wanted a Unix timestamp with zeroes for those values, you'd just do something like:

my ((undef) x 5, $year, $wday, $yday, $isdst) = localtime(time);
my $zeroed_ts = timelocal((0) x 5, $year, $wday, $yday, $isdst);
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