Here's what you need...

sub unweb
{

    if($ENV{QUERY_STRING} eq "")
    {
        read(STDIN, $buffer, $ENV{'CONTENT_LENGTH'});
    }
    else
    {
        $buffer = $ENV{QUERY_STRING};
    }
    @pairs = split(/&/,  $buffer);
    foreach $pair (@pairs)
    {
        ($name, $value) = split(/=/, $pair);
        $value =~ tr/+/ /;
        $value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
        $value =~ s/~!/ ~!/g;
        $FORM{$name} = $value;
    }

    return %FORM;
}
[download]

Have a look at cgi101. That's where I started with CGIs.