G'day ostra,

My guess would be that "check_required items" is supposed to be "check_required_items". Note the second underscore ('_') and that this occurs in two places ("&check_required items ..." and "sub check_required items ...").

You haven't indicated which edition of the book you're referring to (I believe there's quite a few). You haven't provided a link to the book or its example source code (if such exists). Consequently, I'm not in position to check this for you.

It may be a typographical error. It might just be a printing error/defect (look closely: are there possibly faint underscores where I indicated). Also, check online for any published errata.

Finally, what you've "copied" is probably not the same as what you see in the book: I imagine it has at least some reasonable indentation. Unfortunately, this brings into question how close what you typed is to the original: other than whitespace it might be identical; however, my guess at a correction involves whitespace.

check the errata for the book :) Also use diagnostics ( and these checklists Basic debugging checklist , brian's Guide to Solving Any Perl Problem, CGI Help Guide , Troubleshooting Perl CGI scripts ... More generic advice :) On debugging, verify everything, talk to teddybear ... checklists and more , perltrap, perltrap, Common Perl Pitfalls / perltrap)

$ perl -le " sub foo { } "

$ perl -le " sub foo spaces { } "
Illegal declaration of subroutine main::foo at -e line 1.

$ perl -le " use diagnostics; sub foo spaces { } "
Illegal declaration of subroutine main::foo at -e line 1 (#1)
    (F) A subroutine was not declared correctly.  See perlsub.

Uncaught exception from user code:
        Illegal declaration of subroutine main::foo at -e line 1.
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subs don't like spaces in names, or random *stuff* between end of name and { opening brace {

see perlsub for details

see also

chromatics free book Modern Perl a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.
Learn Perl in about 2 hours 30 minutes

#!/usr/bin/perl

use strict;
use warnings;
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#!/usr/bin/perl -l

use strict;
use warnings;

my (@gilligan) = qw(
    red_shirt
    hat
    lucky_socks
    water_bottle
);

check_require_items( 'gilligan', @gilligan );

sub check_require_items {
    use strict;
    use warnings;
    
    my $who = shift();
    my (%whos_items) = map( { $_, 1; } @_ );

    my (@required) = qw(
        preserver
        sunscreen
        water_bottle
        jacket
    );
    
    foreach my $item (@required) {
        unless ( $whos_items{$item} ) {
            print "$who is missing $item.";
        }
    }
}
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