Just select the format you like assigning to $~. Example:

#!/usr/bin/perl

our(@l,$c);
while(@l = getpwent) {
    $~ = $c++ % 2 ? 'foo' : 'bar';
    write;
}

format foo =
left @<<<<<<<<<<<< @<<<<<<<<<<<<
$l[0],$l[7]
.
format bar =
right @>>>>>>>>>>> @>>>>>>>>>>>>
$l[0],$l[7]
.
[download]

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}