Subroutine returning a string warns about "Useless use of a constant in void context" - perl

Question

I have a simple function blurb in a module that returns some text
package Il::NetApp::Dox::FlashCache;
use strict;
use warnings;
no if $] >= 5.018, warnings => "experimental::smartmatch";
use Carp;
use Carp::Heavy;
use Data::Dumper;
use FindBin qw($Bin);
use Il::SysMon::Tools 3.2.1 qw( :debug :special_chars :text_format :help_snippets);
use Il::NetApp::Tools qw( :help_snippets );
# ===========================================================================
# = Texte - ab hier wird übersetzt =
# ===========================================================================
# ===========================================================================
# Markdown Syntax in blurb, extra und examples=>txt!
#
# Verwendbare Variablen in den Texten:
#
# $VERBOSE_HINT = Hinweis -v einzusetzen
#
# ===========================================================================
sub blurb {
q{Checks several metrics of NetApps FlashCache (PAM II).}; # Line 27
}
sub extra {
<<END_EXTRA,
This plugin checks various performance counters of the NetApp-system.
A list of supported counters is printed via the `--counter` switch.
$HELP_DISCOVER_COUNTERS
END_EXTRA
}
#
# Examples: Hier ist jeweils nur txt => zu übersetzen
#
sub simple_examples {
my $examples =
[
{
cmd => q{--explore=counters},
txt => q{List all available and supported counters on the target system.}
},
{
cmd => q{-z hit_percent -w 0 -c 0},
txt => q{Monitor the hitrate for trendanalyses but do not alarm.}
},
]
; # Ende von my $examples =
return $examples;
}
# sub advanced_examples {
# my $examples =
# [
# {
# cmd => q{},
# txt => q{}
# },
# ]
# ; # Ende von my $examples =
# return $examples;
# }
# ===========================================================================
# = ENDE der Texte - ab hier ist nichts mehr zu übersetzen =
# ===========================================================================
1; # return true
On one server we are getting occasional warnings:
Useless use of a constant ("Checks several metrics of "...) in void context at .../lib/Il/NetApp/Dox/FlashCache.pm line 27.
A Perl subroutine returns the value of the last statement executed if it is an expression, and this technique has worked before. I can't reproduce the problem with Perl v5.10.1 or v5.18.2.
The site having these warnings is running Perl v5.16.3
# perl --version
This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi
(with 33 registered patches, see perl -V for more detail)
Could this be a bug in a specific Perl version?

Answer 1

Void context is a context where there's nothing to consume what's been returned.
The warnings occurs where the sub is defined with an empty prototype:
use warnings;
sub blurb () { q(some string) }
blurb();
Can you show NetApp/Dox/FlashCache.pm line 27?

Answer 2

Oh no, finally we have found the reason for that error message "Useless use of a constant" on some customers server. The problem had been introduced by a bug in the distribution chain on the way to the customer.
The code we have sent was (example, reduced):
#!/usr/local/bin/perl -w
use warnings;
use strict;
use feature ":5.10";
say blurb();
say "\n\n\nDESCRIPTION and EXAMPLES\n\n" . extra();
sub blurb {
q{I am the blurb - print me wherever you want me to be shown.};
}
sub extra {
<<END;
Some extra documentation - spanning several lines including indents.
EXAMPLES:
# so something
do_something.pl -H <hostname>
Does something with hostname.
HINT: Da also lag der Hund begraben (German saying, "that's where the dog is buried" translates to "there's the fly in the ointment")
END
}
The code which had actually been run on the customers server and which caused that error was slightly different:
#!/usr/local/bin/perl -w
use warnings;
use strict;
use feature ":5.10";
say blurb();
say "\n\n\nDESCRIPTION and EXAMPLES\n\n" . extra();
sub blurb {
q{I am the blurb - print me wherever you want me to be shown.};
my $status;
}
sub extra {
<<END;
Some extra documentation - spanning several lines including indents.
EXAMPLES:
# so something
do_something.pl -H <hostname>
Does something with hostname.
HINT: Da also lag der Hund begraben (German saying, "that's where the dog is buried" translates to "there's the fly in the ointment")
END
}
The cause for the error-message was the additional line in the blurb-sub.
Conclusions
Beside of fixing the distribution chain future code will be made a bit more robust by either storing the text in a variable and returning that variable later or leaving out the last semicolon which would have thrown a clear error during compile-time and would have saved hours of debugging.
#!/usr/local/bin/perl -w
use warnings;
use strict;
use feature ":5.10";
say blurb();
say "\n\n\nDESCRIPTION and EXAMPLES\n\n" . extra();
sub blurb {
q{I am the blurb - print me wherever you want me to be shown.}
}
sub extra {
my $extra =
<<END;
Some extra documentation - spanning several lines including indents.
EXAMPLES:
# so something
do_something.pl -H <hostname>
Does something with hostname.
HINT: Da also lag der Hund begraben (German saying, "that's where the dog is buried" translates to "there's the fly in the ointment")
END
return $extra;
}
Sorry for the first miss-leading code-example and thanks for all your comments!