use strict;
use warnings;

my %data=();
for my $file (qw /file_1.txt file_2.txt, file_3.txt ... file_n.txt/) {
     open my $FH, "<", $file or die "could not open $file $!";
     while (<$FH>) {
          chomp;
          my ($sz,$sum,@f) = split /\s+/;
          push @{$data{$sz}},$_ for @f;
     }
     close $FH;
}
# ...

##</code><code>##

use strict;
use warnings;

my %data=();
for my $file (@ARGV) {
     open my $FH, "<", $file or die "could not open $file $!";
     while (<$FH>) {
          # ...
     }
     # ...
}
# ...

##</code><code>##

use strict;
use warnings;
use autodie;

my %data=();
while (<>) {
     chomp;
     # ...
}
#...

##</code><code>##

use strict;
use warnings;
use autodie;

my $stat_dir = shift;
my %data = ();
 
{
     local @ARGV = glob ("$stat_dir/*.*");
     while (<>) {
          chomp;
          # ...
     }
     # ...
}