If you want to use a regular expression, I believe that you're going to have to consume the original string, ala:

use strict;
use warnings;
my $count;
# get $input from some place
$input =~ s/^\s+//; # dump leading whitespace
while ($input =~ s/[^\s]+\s+//)
{
  ++$count;
}
++$count if ($input =~ /./); # catch the last word
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but, I think, what you really want is 'split':

my @array = split /\s+/, $input;
$count = scalar (@array);
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