The easiest way to do this is to write your own interface to the formline function making use of the Accumulator ($^A). Look at the perlform man page for more details. As mentioned above, the man page describes a function swrite(). For reference this is it:

sub swrite {
        croak "usage: swrite PICTURE ARGS" unless @_;
        my $format = shift;
        $^A = "";
        formline($format,@_);
        return $^A;
    }
[download]

To actually make use of this is a little harder because you will have to call this function repeatedly as your format changes. This will become clearer once you play with it.

HTH
Simon