if(@array[3..4] =~ /\S/) {
    do stuff;
}
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is the same as:

if($array[4] =~ /\S/) {
    do stuff;
}
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The slice evaluates to the last element. You can try one of these to 'stringify' the array slice first (assuming $" is set to the default):

if("@array[3,4]" =~ /\S/) {
or
if ("@array[3,4]" =~ tr/ \t\n//c) {
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