# pseodocode !!!

use strict; 
use warnings;

use Text::CSV_XS;

# declare early used vars and others
my $TOLERANCE = 0.000001; # you can also use constants in Perl..
my %final_datastructure;

#verify @ARGV filling @files
my @files ..

# define a lookup table for different files
my %lookup_table{
   type_a => \&proc_file_a,
   type_b => \&proc_file_b,
   ...
}


foreach my $file (@files){

  # maybe check readbility? or construct the full path..
  
  process_file ($file);
}


# output the data
out_data(\%final_datastructure);


# the general sub
sub process_file{
  my $fname = shift;
  my $icnt = 0;
  open my $fh, "<:encoding(utf8)", $fname or die "cannot Open $fname : $!";

  # pass the filehandle to specialized subroutines 
  if ($fname eq 'type_a.csv'){
    # pass the filehandle by reference
    proc_file_a(\$fh);
  }
}

# the specialized sub
sub proc_file_a{

  my $fh = shift;
  my $csv = Text::CSV_XS->new ({ binary => 1 }) or
       die "Cannot use CSV: ".Text::CSV_XS->error_diag ();
  
  while (my $row = $csv->getline ($fh)){

    # fill in the destination datastructure..
  
  ...


# sub to output the data
sub out_data{
...