for my $acct ( qw( CAL2345-06 CALI123456-09 FLOR1234567-01 ) ) {
    my $formatted_account_id = format_account_id($acct);
    print "'$formatted_account_id'\n";
}

sub format_account_id {
    my ($acct) = @_;
    my $padding_needed = 14 - length($acct);
    return $acct if $padding_needed < 1;

    $acct =~ m{ \A ([[:alpha:]]+) (.+) \z }msx
        or warn "Bad format: '$acct'" and return $acct;

    return $1 . ( 'X' x $padding_needed ) . $2;
}
[download]

Output:

'CALXXXX2345-06'
'CALIX123456-09'
'FLOR1234567-01'
[download]