You could simplify your code's appearance by defining a utility function like

sub maybe_set_param {
  my ( $self, $param, $default, $value ) = @_;
  $self->session->param( $param => defined $value ? $value : $default 
+);
}
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Then you'd replace the param-setting if-statement with

$self->maybe_set_param( page => $post{ page }, 1 );
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Or, if you always get your param values from this %post hash, then you can simplify things a bit further like this:

sub maybe_set_param {
  my ( $self, $param, $default ) = @_;
  my $value = $self->post->{ $param };
  $self->session->param( $param => defined $value ? $value : $default 
+);
}
# ...
$self->maybe_set_param( page => 1 );
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In the last version, I used an accessor post defined to return a ref to the %post hash.

And from here you can simplify things even further, by making maybe_set_param capable of handling multiple params in one call:

sub maybe_set_param {
  my $self     = shift;
  my %defaults = @_;
  my $post     = $self->post;
  while ( my ( $param, $default ) = each %defaults ) {
    my $value = $post->{ $param };
    $self->session->param( $param => defined $value ? $value : $defaul
+t );
  }
}
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Now you can set various parameters at once like this:

$self->maybe_set_param( page => 1, foo => 15, bar => -1 );
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