if the memory allocated to the hash had not been passed by ref. to the array, would perl have released it's memory?

Yes. Perl uses reference counting to determine if a variable is still in use. As soon as there are no references to a variable, the memory is released back to Perl (but not necessarilly to the operating system). For example,

{
   # Creates a scalar with value 'val1'.
   my $var1 = 'val1';  # 1 reference to val1.
}

# $var1 goes out of scope.
# 0 references to val1.
# No more references to 'val1', so it is released.

my $var2;
{
   # Creates a scalar with value 'val1'.
   my $var3 = 'val2';  # 1 reference to val2.
   $var2 = \$var3;     # 2 references to val2.
}

# $var3 goes out of scope.
# 1 reference to val2.

$var2 = 'val3'  # 0 references to val2. 1 reference to val3.

# No more references to 'val2', so it is released.
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