The pattern doesn't match for every key and value of the hash, causing $2 to be undefined for such cases. To resolve that alone:

sub _find_modules {
    my $dir = shift ;
    my ($inst) = ExtUtils::Installed->new();
    my $packlist = $inst->packlist("My_Package");
    return grep { /.*?\Q$dir\E\w+\.pm$/ and s/(.*?\Q$dir\E)(\w+)(\.pm)
+$/$2/; } %$packlist ;
}
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