The presence of initial spaces forces the empty string as the first element. That effect is suppressed for trailing match, but the initial match is regarded an a separator.

The magical split trims leading and trailing whitespace, so is probably what you want:

$ perl -MData::Dumper -e '
> $str = "             STRING      "; 
> @ar = split(" ", $str);
> print Dumper \@ar;'
$VAR1 = [
          'STRING'
        ];
$
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