I simply thought that this problem might be a good way for me to get my feet wet with the debugger. I thought it might bring me close to perl's machine model.

Yes indeed. I needed to change that line with caller:

my $rvars = \%vars;
## this function is to be debugged using the perl debugger
my $return2 = make_initial_captions($rvars);
say "return2 is $return2";
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Let's consider this. I know that we're encouraged to come up with an SSCCE, but this is not, nor can it easily be made so. It's an html templating system that has evolved according to my needs. I've tried to make a distribution of it, but I couldn't manage how to deal with filesystems on github. What would you use instead of Net::SFTP::Foreign?

This was my effort to be concise. I don't see how the use statements in subroutines affects other modules:

Update: put module source between readmore tags

package utils1;
require Exporter;


our @ISA = qw(Exporter);
our @EXPORT = qw( invert_aoa print_hash print_aoa
highest_number );

sub invert_aoa{
use strict;
use warnings;
use 5.010;

my $a = shift;
my @AoA = @$a;
my $k = $#AoA;
#say "k is $k"; 
my @BoB;
    for my $i ( 0 .. $#AoA ) {
        my $aref = $AoA[$i];
        my $x = $#{$aref};
        #say "x is $x";
        for my $j ( 0 .. $#{$aref} ) {
            $BoB[$j][$i]= $AoA[$i][$j];
        }
    }
my $b = \@BoB;
return $b;
}
sub print_aoa{
use warnings;
use 5.011;

my $a = shift;
my @AoA = @$a;
    for my $i ( 0 .. $#AoA ) {
        my $aref = $AoA[$i];
        for my $j ( 0 .. $#{$aref} ) {
            print "elt $i $j is $AoA[$i][$j]\n";
        }
    }
return $a;
}

sub highest_number{
use strict;
use File::Basename;
use Cwd;

my ($aref, $filetype, $word) = @_;
my $number;
my @matching;
my $ext = ".".$filetype;
push (@matching, 0); #min returned value
for my $file (@{$aref}) {
#print "file is $file\n";
if ($file =~ /^$word(\d+)$ext$/){
   #print "matching is $file\n";
   push (@matching, $1);
   }
}
@matching = sort @matching;
my $winner  = pop @matching;
return $winner  
}

sub print_hash{
   use 5.011;
   my $hash_ref = shift;
   ## 10-15-18 adding sorted keys
   print "subroutine says this is your hash: \n";
   my %hash = %$hash_ref;

    foreach my $name (sort keys %hash) {
        say "key: $name, value: $hash{$name}\n";
    }
}


1;
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Question: what use statements can be removed from everything I've posted?

This routine has a breakpoint now:

sub make_initial_captions {
  use 5.016;
  use warnings;
  use POSIX;
  use Path::Tiny;
  use Encode;
  use open OUT => ':encoding(UTF-8)', ':std';
  use Data::Dumper;

  my $rvars = shift;
  my %vars  = %$rvars;

  #print Dumper $rvars;
  
  my $image_path   = $vars{"to_images"};
  my $caption_path = $vars{"eng_captions"};
  # put a break point here
  $DB::single = 1;
  return "nothing yet";
}
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, and, I got to see some values at this mark:

$ perl -d 1.debug.11.pl

Loading DB routines from perl5db.pl version 1.55
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

Subroutine debug1::getcwd redefined at /usr/share/perl/5.30/Exporter.p
+m line 66.
 at template_stuff/debug1.pm line 400.
    debug1::BEGIN() called at template_stuff/debug1.pm line 400
    eval {...} called at template_stuff/debug1.pm line 400
    require debug1.pm called at 1.debug.11.pl line 4
    main::BEGIN() called at template_stuff/debug1.pm line 400
    eval {...} called at template_stuff/debug1.pm line 400
main::(1.debug.11.pl:12):    my $ts          = "template_stuff";
  DB<1> n                                                             
+          
main::(1.debug.11.pl:13):    my $images      = "aimages";
  DB<1> c                                                             
+          
title is 1.debug.1
path1 is /home/hogan/6.scripts/1.debug.1
abs is /home/hogan/6.scripts/1.debug.1/1.debug.11.pl
debug1::make_initial_captions(template_stuff/debug1.pm:43):
43:      return "nothing yet";
                                                          
...
  DB<3> p $vars{to_images}                                            
+          
/home/hogan/6.scripts/1.debug.1/template_stuff/aimages
  DB<4> p $vars{eng_captions}                                         
+          
/home/hogan/6.scripts/1.debug.1/template_stuff/captions
  DB<5> c                                                             
+          
return2 is nothing yet
ini path is /home/hogan/Documents/html_template_data/6.values.ini
...[normal execution]
return is 1.debug.13.html
http://www.merrillpjensen.com/perlmonks/1.debug.13.html
Debugged program terminated.  Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
  DB<5> q                                                             
+          
$
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It then executed to the end and behaved.