Ok, looked a little longer at your code and figure I see what you are trying to do. The following probably addresses your immediate problem:

use strict;
use warnings;

use XML::Twig;

use vars qw (%PARAMS);
do 'noname1.pl';

for my $key (keys %PARAMS) {
    my $val = $PARAMS{$key}; # Need var because sub turns it into a cl
+osure
    
    $PARAMS{$key} = sub {upd_param (@_, $val)};
}

my $twigT= new XML::Twig(
    twig_roots   => \%PARAMS,
    twig_print_outside_roots => 1,              
    );

$twigT->parse (do {local $/; <DATA>});
$twigT->purge; #probably not needed

sub upd_param {
    my ($t, $param,$new_param) = @_;
    my $item = defined($new_param)? $new_param:$param->text;
    
    $param->set_text($item);                # change it
    $param->print;                          # print updated value
    $t->purge;                              # free the memory
}

__DATA__
<task>
    <params>
        <CellRefinementLevel></CellRefinementLevel>
    </params>
</task>
[download]

Prints:

<task>
    <params>
        <CellRefinementLevel>7</CellRefinementLevel>
    </params>
</task>
[download]

given a file noname1.pl containing:

$PARAMS{'CellRefinementLevel'}=7;
$PARAMS{'GridCellSizeInXDirection'}=100;
$PARAMS{'GridCellSizeInYDirection'}=100;
[download]

However the same mapping could be provided using a rather less frightening text file containing:

CellRefinementLevel=7
GridCellSizeInXDirection=100
GridCellSizeInYDirection=100
[download]

if the use vars and do lines are replaced with:

@ARGV = 'noname.txt'; # omit if config file passed on the command line
my %PARAMS = map {chomp; split '='} grep {length} <>;
[download]