Take choroba's length suggestion to heart; it's probably the best way to go...

But, for purposes of generalizing your modification procedure (which is both overkill and underkill, try this for size:

#!/usr/bin/perl
use strict;
use warnings;
# 927370 - OP seeks to insert 'X' enough times to pad to len=14 AFTER 
+the LETTERS
use 5.012;

# NB: First account in this array needs more than one 'X'tension...
my @account = qw/CAL2345-06 CALI123456-09 FLOR1234567-01/;

for my $account(@account) {
    my $total_count = 0;
    while ($account =~ m/./g) {
        $total_count++ ;
    }
    say "At ln 15, |$account| has $total_count characters";

    while ($total_count < 14) {        
        substr($account, 4, 0) = 'X';
        $total_count++;
    } continue {                   # loop till $total_count == 14 !
        say "At ln 21, |$account| has $total_count characters";
    }
    next;
}
[download]

Execution and output:

C:\>927370.pl
At ln 15, |CAL2345-06| has 10 characters
At ln 21, |CAL2X345-06| has 11 characters  <!-- first insert, X in wro
+ng position
At ln 21, |CAL2XX345-06| has 12 characters
At ln 21, |CAL2XXX345-06| has 13 characters
At ln 21, |CAL2XXXX345-06| has 14 characters <!-- but we do get to 14,
+ despite short orig.
At ln 15, |CALI123456-09| has 13 characters
At ln 21, |CALIX123456-09| has 14 characters <!-- 14 after extension
At ln 15, |FLOR1234567-01| has 14 characters <!-- 14 in original array

C:\>
[download]

hth

Extensively updated: pasted similar output and code but with mismatching ln numbers.