That's not quite correct...

IO::Prompt returns an object, in that object bool is overloaded, and bool sets $_. (which is run for each iteration of the while to decided if it's a true or false value)

Which means that while ($z = prompt "") { will set $_ and $z. This is a bad idea. What if the code is used in an inner loop? when the outer loop uses $_ for example?

If one wants to get the same behaviour then it would be enough to do a simple assignment in the subroutine to $_ and return the value of $_.

Example code that is similar to the code in IO::Prompt

package Example;

use overload q{bool} => sub { $_="def"; }; 

sub new { bless {}; } 
sub test { return $_[0]; } 

package main;
my $z=new Example;
$_="ghi";
while ($m = $z->test()) { 
  print; 
  print $m; 
  last; 
};
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Update: added relevant part of IO::Prompt