The original hash was getting changed because all your references pointed to that hash. The thing to remember is that when you derefence a reference, you get the original data structure back, not a copy of the structure. To get a copy, you need to make the copy when you create the reference.

my %hash = (snark => 'snark');     # hash
my $ref = \%hash;                  # ref to hash
surprise($ref);                    # pass ref to hash

sub surprise {
  my $ref = shift;                 # copy ref to hash
  $ref->{snark} = 'boojum';        # change ref'ed hash
}

print "snark => $hash{snark}\n";   # print original hash

__END__
snark => boojum
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