use warnings;
use strict;

my %std_data;
my $std_names  = 'names.txt';
my $std_grades = 'grades.txt';

  ## call subroutrine read_file, passing different file name  
@std_data{ @{ read_file($std_names) } } = @{ read_file($std_grades) };

print $_,q{,},$std_data{$_},$/ for sort { $a cmp $b } keys %std_data;

sub read_file {
    my ($file) = @_;
    my $data = [];
    open my $fh, '<', $file or die "can't open file:$!";
    while ( defined( my $line = <$fh> ) ) {
        chomp $line;
        push @{$data}, $line;
    }
    close $fh or die "can't close file:$!";
    return $data;
}