If this was some generic checksum routine, you could add some validation (5.8+)

sub checksum {
    my $s = shift;
    utf8::downgrade($s, 1)
        or croak("Wide character in subroutine entry");
    return unpack("%32C*", $s) % 65535;
}
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For earlier backwards compatibility, that would be

sub checksum {
    my $s = shift;
    if (defined(&utf8::downgrade)) {
        utf8::downgrade($s, 1)
            or croak("Wide character in subroutine entry");
    }
    return unpack("%32C*", $s) % 65535;
}
[download]

Update: Switched utf8->can() (method check) for defined(&) (sub check).