use strict;
use warnings;

my @A = (0) x 20;

PrintA(); 
ChangeArray5(\@A);
PrintA(); 

sub ChangeArray5
{
  my $B = shift;       # I don't understand this, but it works :P

  foreach my $X (@$B)
  {
    $X = 5;
  }
}

sub PrintA { print "\n".join('', @A); }

__END__
Prints:

00000000000000000000
55555555555555555555