Using  tr/// as davido suggests can very efficiently count characters and also exclude characters from a count with the  /c modifier (see  tr/// in the Quote-Like Operators section of perlop):

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qq(pp);
 ;;
 my $entab = 4;
 ;;
 my @lines = (
   qq{This line has no tab.\n},
   qq{\tThis line has tab at the beginning.\n}
   );
 ;;
 for my $line (@lines) {
   my $rend = ($line =~ tr/\t//c) + ($entab * $line =~ tr/\t//);
   printf qq{%s renders as %d chars long \n}, pp($line), $rend;
   }
"
"This line has no tab.\n" renders as 22 chars long
"\tThis line has tab at the beginning.\n" renders as 40 chars long
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