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$ ./3.sscce.pl 
1..1
number is 1
...
# Looks like your test exited with 255 before it could output anything
+.
$ cat 3.sscce.pl

#!/usr/bin/perl -w
use 5.011;
use Path::Tiny;
...


__END__

{
  my $subset = getsubset( $data, "R2C5:R4C8" );
  print_aoa $subset;
...
  $subset->[1][2] = 'X';
  say "substitutes X";
}