As another poster mentioned, you can use %INC and File::Basename to identify the filesystem path where acme.pm was found:

use File::Basename 'dirname';

sub get_config {
   my $filename = dirname($INC{'acme.pm'}) . "/$_[0]";
   croak "Can't find $filename" unless -f $filename;
   ...
}
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Another option is to simply walk @INC, or some other "config file path" of your choice and find this file:

sub find_config {
   my $filename = shift;
   foreach (@_) {
      return "$_/$filename" if -f "$_/$filename";
   }
   return;
}

my $filename = find_config("acme.conf", @INC, "/also/search/here", "/a
+nd/here")
   or die "Couldn't find acme.conf!";
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