printf( "A float: %12.4f$/", $root_beer );

##</code><code>##

my $fmt = fmt_num_or_blank( $width, $data);
printf( "A float: $fmt$/", $root_beer );

##</code><code>##

#  Mnemonic: N nothing for nought nor null
printf( "A float: %N12.4f$/", $root_beer );