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#!/usr/bin/perl

use DB_File;
...
$hash{'thisvar'} = param('webvar'); # modifies from form value

untie %hash;

$filename = '/path/to/config/file';
open(CONF,"<$filename");
while(<CONF>){
...
open(CONF, ">$filename");
print CONF "$_\t$config{$_}\n" for keys %config;
close(CONF);