Right, that case works because print itself is doing the conversion. If $# is set then each element passed to print is looked at to see if it can be converted to an IV or an NV. If it can, it's formatted using $#. Spaces don't matter when considering if it's numeric, so "  1.23456  " doesn't look any different than a numeric 1.23456 (and in fact the spaces will be stripped before the conversion is done).

$  perl -le '$#="%0.2f";$a=1.2345;print "|", "  $a  ", "|";'
|1.23|
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However in the other cases, there's more than spaces and digits (and decimals, and whatnot that perl recognizes as a numeric) so it's not seen as an IV or NV and not treated to the $# munging. See Perl_do_print in doio.c for the complete skinny on what exactly happens.