I had the same problem and came across a solution that gcc uses. For various reasons this is expressed as a bash script and a more recent version is probably abailable. I leave the conversion to Perl as an exercise for the reader:

    # Short circuit this function if we know which system we are on
    if [ $OS_SYSTEM_REALLY ]; then
        echo "${OS_SYSTEM_REALLY}"
        return
    fi

    # First get the values for this machine

    # Code lifted from config.guess in the gcc software
    OS_UNAME_M=`(uname -m) 2>/dev/null` || OS_UNAME_M=unknown
    OS_UNAME_S=`(uname -s) 2>/dev/null` || OS_UNAME_S=unknown
    OS_UNAME_R=`(uname -r) 2>/dev/null` || OS_UNAME_R=unknown
    OS_UNAME_V=`(uname -v) 2>/dev/null` || OS_UNAME_V=unknown
    OS_UNAME="${OS_UNAME_M}:${OS_UNAME_S}:${OS_UNAME_R}:${OS_UNAME_V}"

    # Note: order is significant - the case branches are not exclusive
+.
    case "${OS_UNAME}" in
        sun4*:SunOS:5.[012345]*:*)
            echo "sol24"
            return ;;
        sun4*:SunOS:5.[6789]*:*)
            echo "sol26"
            return ;;
        sun4*:SunOS:[7891]*:*)
            echo "Unknown Solaris version" >&2
            echo "Try setting \$OS_SYSTEM_REALLY to sol26" >&2
            echo "unknown"
            return ;;
        sun4*:SunOS:*:*)
            echo "sun"
            return ;;
        *:AIX:1:4)
            echo "aix41"
            return ;;
        *:IRIX*:5.*:*)
            echo "sgi"
            return ;;
        *:IRIX*:6.[2345]:*)
            echo "irix62"
            return ;;
        9000/[78]??:HP-UX:*:*)
            echo "hp"
            return ;;
        i[345678]86:Linux:*:*)
            echo "linux"
            return ;;
        sparc:Linux:*:*)
            echo "ulinux"
            return ;;
        *:*:*:*)
            # If we get here then we have something weird
            echo "Unknown system \"${OS_UNAME}\"" >&2
            echo "Try setting \$OS_SYSTEM_REALLY to one of sun, sol, h
+p etc" >&2
            echo "unknown"
            return ;;
    esac
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