use CGI qw/:standard/;

$file = "bar";

defined param(selected)
   ? param(selected, param(selected) . ":$file") 
   : param(selected , $file) ;

print join ( "", map {
                       hidden (
                                -name    => $_,
                                -default => $param{$_},
                              )
                       }  param
            )

##</code><code>##

C:\test>perl param.pl selected=food
<input type="hidden" name="selected" value="food:bar" />
C:\test>perl param.pl
<input type="hidden" name="selected" value="bar" />