Further to RMGir here:

If you want to pass a local copy, specify that:

pass1({ params => {%$hashref} });  # {%$hashref} makes a copy of the hash '$hashref'
                                   # points to and creates a new ref to it.

... and stevieb here:

You need to "copy" the reference in order to break the link:

my $params = { %{ $self->{params} } };

There's a pitfall here. Be aware that the "copy" above is only a shallow copy. Any references nested within the referent of the reference being shallow-copied still allow complete read/write access to their referents.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $hashref = { foo => { bar => 'nice' } };
 dd 'before pass1: ', $hashref;
 ;;
 pass1({ params => { %$hashref } });
 ;;
 dd 'after pass1: ', $hashref;
 ;;
 ;;
 sub pass1 {
   my $self = shift;
   my $params = $self->{params};
   dd 'in pass1, before assignments ', $params;
   ;;
   $params->{fizz} = 'fazz';
   $params->{foo}{bar} = 'KaPow';
   dd 'in pass1 after assignments ', $params;
   }
"
("before pass1: ", { foo => { bar => "nice" } })

("in pass1, before assignments ", { foo => { bar => "nice" } })

(
  "in pass1 after assignments ",
  { fizz => "fazz", foo => { bar => "KaPow" } },
)

("after pass1: ", { foo => { bar => "KaPow" } })
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Bottom line: references be tricky. (Update: See perlref, perlreftut, maybe also perldsc.)