"Twice as fast" seems like a lot for in memory operations when there are also disk accesses.

Yes, I thought so too. Looks like my data set was so large it ate into swap. :-)

Re-running with a smaller data set still shows quite a decent speed up, however. Here's my bench and results:

#!/usr/bin/env perl 
use strict;
use warnings;
use Benchmark 'cmpthese';

my $size = 50_000_000;
my @big = (rand () x $size);

cmpthese (10, {
    'interp' => 'interp ()',
    'Eily'   => 'eily ()',
    'OFS'    => 'ofs ()',
});
exit;

sub interp {
    open FH, '>', 'mergedlogs.txt' or die "can't open mergedlogs.txt: 
+$!";
    local $| = 0;
    foreach (@big) { print FH "$_\n"; }
    close FH;
}

sub eily {
    my $output_file = "mergedlogs.txt";
    open my $output_fh, ">", $output_file or die "Can't open $output_f
+ile: $!";
    local $| = 0;
    local $\ = "\n";
    foreach (@big) { print $output_fh $_; }
    close $output_fh;
}

sub ofs {
    my $output_file = "mergedlogs.txt";
    open my $output_fh, ">", $output_file or die "Can't open $output_f
+ile: $!";
    local $| = 0;
    local $\ = "\n";
    local $, = "\n";
    print $output_fh @big;
    close $output_fh;
}
[download]

       s/iter interp   Eily    OFS
interp   1.83     --   -35%   -35%
Eily     1.20    53%     --    -1%
OFS      1.19    54%     1%     --
[download]