with length().. 
$str=123;
$fu = length($str);
print $fu,"\n";

[download]

hm..
To count the digits in an alphanum string.. 
perl -e '$str="fff123"; $fu = ($str =~ tr/[a-z]//c); print $fu,"\n";'

[download]
I know, it's not what you asked, just trying to complicate things needlessly:)
updated, pls see following reply

In reply to Re: count digit in a numeric value by hsinclai
in thread count digit in a numeric value by wusphere

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