Your code is fine, but $sentence has no value.

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump pp);

my $sentence = 'aaaa bbb  ccc';

my @words = split(/\s+/, $sentence);

my $i;
for($i=0;$i<scalar (@words);$i++)
{
    print "$i: " . $words[$i] . "\n";
}
[download]

Produces

0: aaaa
1: bbb
2: ccc
[download]