If you are using PLP, you can use the %get and %post hashes (which are combined in %fields, but IMO it's bad to mix the two).

<form method=get>
<input name=test>
</form>
<:
    if ($get{test} ne '') {
        print "Your input: $get{test}"
    }
:>
[download]

Juerd # { site => 'juerd.nl', plp_site => 'plp.juerd.nl', do_not_use => 'spamtrap' }