Help: hash key pass to subroutine as string failed

[fenghuo]

I have a sub expand() to expand the path

sub expand($%) {

    my $self = shift;
    my ($string, %params) = @_;

    # Perform parameter checks.
    die("Mandatory parameter 'string' not defined.") unless (defined $string);
    //other handling
}

The followng call fails
 
sub foo {
      my $files = {
        'test.in'    => 'test.out'
    };
   
    expand($_) 
        foreach ( keys( %{$files} ) );
}
saying the 'string' not defined.
While the following codes work:
sub foo {
      my $files = {
        'test.in'    => 'test.out'
    };
   
   foreach my $key ( keys %{$files} )  {
            expand (+$key); #expand ($key) also fails
   }
}

Could some one give me a hint why? I am newbie for Perl.
Thanks!

[KevinADC]

As shawn on another forum said, don't use prototypes, instead of :

sub expand($%) {

use a sub:

sub expand {

[fenghuo]

Actually I tried to remove the prototype as suggested, but still failed to pass the correct string value of the key.
the following methods work as well:
method 1:

while ( my ($key,$value)  = each (%{$files}) ){
      expand($key);
    }

methods 2:

foreach my $key (keys  %{$files}  ) {
      my $s = sprintf("%s",$key);
      expand($s);
   }

So I suspect foreach implmentation has sth. tricky.
Any suggestion?

[KevinADC]

See other forum