defined $param{selected} 
        ? ( $param{selected} .= ":$file" )
        : ( $param{selected} = $file );

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

##</code><code>##

perl -le 'use CGI qw/ :standard /; print hidden( -name => "foo", -default => "foo:bar" )'