This is close to how I quite often do this very thing:

my $format = "Hello there %3d!";
my @data = (123);

if (defined $pi) # if we've calculated pi ...
{
  $format .= " %5.1f";
  push @data, $pi;
}

printf "$format\n", @data;
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Very useful to be able to do this. The above example specifically doesn't seem too useful, but the overall idea is quite useful. At least to me.