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;
}
# ...