I'd use tie() instead. This module will untie the variable and replace it with its value the first time the variable is fetched.

package Tie::Scalar::Lazy;

# usage:
#   use Tie::Scalar::Lazy;
#   lazy $value => \&expensive_function;

require Exporter;
@ISA = qw( Exporter );
@EXPORT = qw( lazy );

sub lazy {
  tie $_[0], __PACKAGE__, \$_[0], $_[1];
}

sub TIESCALAR {
  my ($class, $sref, $cref) = @_;
  bless [ $sref, $cref ], $class;
}

sub FETCH {
  my $self = shift;
  my $value = $self->[1]->();
  untie ${ $self->[0] };
  ${ $self->[0] } = $value;
}

1;
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