G'day LittleJack,

From your post yesterday (Starting out with Dancer and falling at the first hurdle) you're using Dancer2. The link you've provided is for Dancer documentation; the Dancer2 equivalent is https://metacpan.org/dist/Dancer2/view/lib/Dancer2/Manual/Deployment.pod#With-Plack. I had a quick look and can't see any substantial differences; however, it's better to use the right version of the documentation.

"Which perl and which libraries is my Apache (2.4.7) trying to use?"

Either modify your app.psgi, or write a new one, which outputs various version (and perhaps other) information. A very rough, command line example:

$ perl -e '
    use List::Util;
    print "Perl version: $]\n";
    print "List::Util version: $List::Util::VERSION\n";
'
Perl version: 5.034000
List::Util version: 1.56
[download]

Other suggestions:

• Check in your Apache logs for some hints.
• Check which version of mod_perl or mod_perl2 you have installed.
• Do you have any instances of Apache::* that should be Apache2::*.
• Specify a minimum Perl version; e.g. 'use 5.020;'.
  • The first version of perldoc which mentions 'pairmap' and 'pairgrep' is for Perl v5.20: https://perldoc.perl.org/5.20.0/List::Util.
• Specify a minimum List::Util version; e.g. 'use List::Util 1.29 qw{...};'.
  • See https://metacpan.org/dist/Scalar-List-Utils/changes which indicates 'pairmap' and 'pairgrep' were introduced in v1.28 (with a bug) and fixed in v1.29.

Note that I'm attempting to guess what your app.psgi and Apache config look like. Something more concrete from you would be helpful.