I believe this will work:

#EARLIER
$self->param( 'sql'   => \%sql );

#LATER
$self->param( 'sql' )->{page_id} = 45;
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Where the concept is that the value of the 'sql' param is a hash reference, so you just need to retrieve that and work with it as if it you had done something like my $href= \%sql;
In the line above, it simply deferences the return value of $self->param( 'sql' ) and sets the page_id key to 45.