use strict;  
use warnings;
use Benchmark qw(cmpthese);

my $line = join (',', 1 .. 1000) . "\n";
my $data = $line x 100;

cmpthese (
    -1,
    {
        array => \&asArray,
        str   => \&asStr,
    }
);


sub asArray {
    my $outStr;
    
    open my $in, '<', \$data;
    open my $out, '>', \$outStr;
    
    print $out join ',', map {s/$/a/; $_} split ',' while <$in>;
    
    return $outStr;
}


sub asStr {
    my $outStr;
    
    open my $in, '<', \$data;
    open my $out, '>', \$outStr;
    
    while (<$in>) {
        s/(?=,)|\n/a/g if /\S/;
        print $out $_;
    }
    
    return $outStr;
}