I'd first try to figure out why $_->[4] is undefined, or if it is, don't try testing its value or interpolating it into a string. But this should prevent the test that causes the warning without resorting to disabling warnings:

if(
        defined $_->[4] 
    and $_->[4] ne ""
) {
    $float = join ',', @{$_}[1..4];
}
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Note: The $_->[4] notation is functionally equivilant to, but IMHO easier to read than ${$_}[4]. I also happen to like using join on an array slice instead of typing out "${$_}[1],${$_}[2],${$_}[3],${$_}[4]", but that's not crucial to fixing your problem. The key is the defined test.