Taking things in backwards order:

• 5/ What did you search for when you checked CPAN for prior art?

  Ask yourself what someone interested in this stuff is most likely to search for and chose a name space and a name that is a good fit for that search. Also make sure that the key words appear in the POD.

  So far as I can find, at present Sport doesn't feature on CPAN (seems a large omission), nor does any sport related analytics.

• 4/ Absolutely use helper functions, but see comments later.
• 3/ Absolutely use error checking on input, and anywhere else that will catch problems early rather than late. See comments below.
• 2/ See comments below.
• 1/ See comments below.

After a quick glance at your code there are a few things I'd change. First off, make it OO. That way you don't need to pass back a mess of data that your client is just going to have to pass back in elsewhere. It also makes dealing with PDL/non-PDL handling tidier.

Handle errors using exceptions (die and eval). That makes dealing with errors in an appropriate place much easier for your code and for the caller's code.

With OO code helper functions are just something provided by the object.

A partial implementation of your code as OO might look like:

use strict;
use warnings;

package Sport::Analytics::SRS;

return 1; # madule returns true for successfull load

sub new {
    my ($class, %params) = @_;

    # parameter validation here. die on failure.

    # Test for PDL
    $params{havePDL} = eval {
        require 'PDL';
        PDL->import ();
        1;
    };

    return bless \%params, $class;
}


sub srs_full_matrix {
    my ($self, %options) = @_;

    die "The module PDL must be available for srs_full_matrix use."
        if !$self->{havePDL};

    $options{epsilon} ||= 0.001;
    $options{maxiter} ||= 1000000;
    $self->{srs} = $self->{mov}->copy ();

    my $oldsrs = $self->{srs}->copy ();
    my $delta  = 10.0;
    my $iter   = 0;

    while ($delta > $options{epsilon} and $iter < $options{maxiter}) {
        my $wt   = 1.0 / sumover ($self->{played});
        my $prod = $self->{srs} x $self->{played};
        my $sos  = $wt * $prod;
        $self->{srs} = $self->{mov} + $sos;
        $delta = max (abs ($self->{srs} - $oldsrs));
        $oldsrs = $self->{srs}->copy ();
        $iter++;
    }

    print "iter  = $iter\n"                if $options{debug};
    print "epsilon  = $options{epsilon}\n" if $options{debug};
    printf "delta = %7.4f\n", $delta if $options{debug};
    my $offset = sum ($self->{srs});
    $self->{srs} -= ($offset / $self->{mov}->nelem);
    return $self->{srs}->slice (":,(0)");
}


sub loadData {
    my ($self, $games) = @_;
    for (@$games) {
        my ($visitor, $visit_score, $home_team, $home_score) = split "
+\,", $_;
        my $diff = $home_score - $visit_score;
        $self->{team}{$visitor}{games_played}++;
        $self->{team}{$home_team}{games_played}++;
        $self->{team}{$visitor}{points} -= $diff;
        $self->{team}{$home_team}{points} += $diff;
        push @{$self->{team}{$visitor}{played}},   $home_team;
        push @{$self->{team}{$home_team}{played}}, $visitor;
    }
    my $total_team = scalar keys %{$self->{team}};

    $self->{played} = zeroes ($total_team, $total_team);
    $self->{mov} = zeroes ($total_team);
    my $ii = 0;
    for (sort keys %{$self->{team}}) {
        my $team_diff =
            $self->{team}{$_}{points} / $self->{team}{$_}{games_played
+};
        $self->{team_map}{$_} = $ii;
        $self->{mov}->set ($ii, $team_diff);
        $ii++;
    }

    for (keys %{$self->{team}}) {
        my $i = $self->{team_map}{$_};
        for my $opp (@{$self->{team}{$_}{played}}) {
            my $j = $self->{team_map}{$opp};
            my $a = $self->{played}->at ($i, $j);
            $a++;
            $self->{played}->set ($i, $j, $a);
        }
    }
}
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so the module might be used like:

use Sport::Analytics::SRS;

eval {
    my $analysis = Sport::Analytics::SRS->new ();
    
    $analysis->loadData ();
    $analysis->fullMatrix ();
    ...
} or do {
    die "Analysis failed: $@\n";
};
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