Look into using printf.

printf "Customer Name: %-24s Number of Books checked out: % 2d\n", @it
+ems[0,5];
[download]

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.

WRT printf, see sprintf for a wealth of format specifiers.

catfish1116:

At first, I guessed that formatting problem is that the data lines contain whitespace which caused that first field to blow up. I was going to suggest that you tell split to ignore whitespace like the following:

use strict;
use warnings;
my $line = <DATA>;
  while (defined($line = <DATA>)) {
    chomp($line);
    my @items = (split /\s*:\s*/, $line);
    print "Customer Name: $items[0]  \t  Number of Books checked out: 
+$items[5]\n";
  }

__DATA__
customer:field1:field2:field3:field4:num_checked_out
wilma flintstone     :1:m:n:o:3
barney rubble:1:e:i:o:5
Sam Slate   :1:3:2:1:0
Great Gazoo:1:1:2:3:2
Charlie Harper:1:a:b:c:4
Neil Gruesome:1:x:y:z:6
[download]

But when I ran the script, I got the same result you did:

$ perl ex_split_ign_whitespace.pl
Customer Name: wilma flintstone           Number of Books checked out:
+ 3
Customer Name: barney rubble      Number of Books checked out: 5
Customer Name: Sam Slate          Number of Books checked out: 0
Customer Name: Great Gazoo        Number of Books checked out: 2
Customer Name: Charlie Harper     Number of Books checked out: 4
Customer Name: Neil Gruesome      Number of Books checked out: 6
[download]

That's when I looked at your print statement: You're using a tab to try to align the columns. But a tab simply tells the print statement to go to the next column that's a multiple of 8. When your input data has width differences, you'll find that a field that's a shade too long pushes everything over eight more characters.

So the solution is to use printf with a width specifier (as tobyink mentioned), or the format statement (as hippo suggested) or pack or some such. I just tweaked it to use printf since it was the simplest change:

use strict;
use warnings;
my $line = <DATA>;
  while (defined($line = <DATA>)) {
    chomp($line);
    my @items = (split /\s*:\s*/, $line);
    printf "Customer Name: %-16s    Number of Books checked out:%3u\n"
+,
        $items[0], $items[5];
  }

__DATA__
customer:field1:field2:field3:field4:num_checked_out
wilma flintstone     :1:m:n:o:3
barney rubble:1:e:i:o:5
Sam Slate   :1:3:2:1:0
Great Gazoo:1:1:2:3:2
Charlie "the Angel" Harper:1:a:b:c:4
Neil Gruesome:1:x:y:z:1026
[download]

In this version, I added a width specifier to both the customer name *and* the number of books checked out. Notice that the columns line up correctly *unless* one of the data items is too long to fit in the field (the last two lines).

$ perl ex_split_ign_whitespace.pl
Customer Name: wilma flintstone    Number of Books checked out:  3
Customer Name: barney rubble       Number of Books checked out:  5
Customer Name: Sam Slate           Number of Books checked out:  0
Customer Name: Great Gazoo         Number of Books checked out:  2
Customer Name: Charlie "the Angel" Harper    Number of Books checked o
+ut:  4
Customer Name: Neil Gruesome       Number of Books checked out:1026
[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.

#!/usr/bin/perl

use strict;
use warnings;

print ToTitleCase('barney rubble');
exit;

############################################
#                                 v2019.12.4
# Capitalizes the first letter of every word,
# and returns a new string.
# Usage: STRING = ToTitleCase(STRING)
#
sub ToTitleCase
{
  my $S = defined $_[0] ? lc($_[0]) : '';
  my ($i, $p, $BREAK) = (-1, 0);
  while (++$i < length($S))
  {
    ($BREAK = index(" .,:;!?/\\()[]{}<>|-+=\t\n\r\xFF", substr($S, $i,
+ 1))) >= 0
      or $p < 0
      or ($p = vec($S, $i, 8)) > 122
      or vec($S, $i, 8) = $p & 223;
    $p = $BREAK;
  }
  return $S;
}
############################################
[download]

A slightly more Perlish alternative:

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

print title_case('barney rubble');
exit;

sub title_case {
    $_ = shift;
    s/\b(\w)/\U$1/g;
    return $_;
}
[download]

See also the FAQ.

Side note: Title case is an editorial term and procedure that is a bit more complicated than capitalizing words, the spelling of names does not have to constrain to any such rules, and uc/lc are not always uniform or roundtrippable operations outside of basic Roman letters.