In your case it will work like this

# perl -MData::Dumper -e '
> $str = "             STRING      ";
> $str =~ s/\s//g;
> push(@ar,$str);
> print Dumper \@ar;'
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You will get

$VAR1 = [
          'STRING'
        ];
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Is this what you want?
But if you want to use split then use it as Zaxo told you