and here's a different style solution for you.

Note the use of use strict forcing me to decalre my variables.

#!/perl -w
use strict;

print"key in your string :";
chomp( my $string_name=<STDIN>);
my $low_string = "a"x length($string_name);
my $count = 0;

while($low_string ne $string_name) {
    $low_string++;
    $count++;
}

print"$string_name is the $count th value starting from $low_string\n"
+;
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