An approach using format since that's in the title of your question.

#!/usr/bin/env perl 
use strict;
use warnings;

my @input = split /\n/, <<EOT;
wilma flinstone:3
barney rubble:5
Sam Slate:0
Great Gazoo:2
Charlie Harper:4
Neil Gruesome:6
EOT

for my $line (@input) {
    my @fields = split /:/, $line;
    format =
Customer Name: @<<<<<<<<<<<<<<<<<< Number of Books checked out: @<
               $fields[0],                                      $field
+s[1]
.
    write;
}
[download]

Consider other options such as Template, printf, Text::Table, etc.