There are several ways you can do this, but the following is reasonably efficient, and hopefully your parameter list is relatively small anyway:

    my $q = new CGI;
    my %vars = $q->Vars; # Fetch all CGI parameters
    my %db_vars = pop_defaults(); # Fetch from database
  
    # Override defined values from database
    $vars{$_} = $db_vars{$_} for (keys %db_vars);

    # Fetch all defaults from the DB and return as a hash ref
    sub pop_defaults { my $r = { }; ... return $r }
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You can also simply loop through the params:

    $vars{$_} = $db_vars{$_} // $q->param($_) for ($q->param);
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Finally, if you prefer to merge hash slices:

    %vars{keys %db_vars} = values %db_vars;
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