Assuming the string is only numerals and isn't all zeros (which could be checked for separately), here's a method that uses coercion of a string into a number.

use strict;
use warnings;

for my $Str1 ( "987654", "087654", "007654", "000654", "000054", "0000
+04" ) {
        my $Index = length($Str1) - length($Str1 + 0) + 1;
        print "$Str1  pos=$Index\n";
}

__END__
987654  pos=1
087654  pos=2
007654  pos=3
000654  pos=4
000054  pos=5
000004  pos=6
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