my %stacks;

my $CurrentArrayRef = $Stacks{nonexistant} || []; 
# Line above CREATES a ref to an empty array if the stack was undefined.
#It returns a ref to the contents if there were any.

push @$CurrentArrayRef, "New Value"; 
 ... more code ...

# Now (re)store the contents into the stack
$Stacks{nonexistant} = $CurrentArrayRef;