For what it's worth, here's my take on the situation:

#!/usr/bin/perl -w
use strict;

my (@data, $i);
my @logfile = <DATA>;
for (@logfile) {
    /blahblah/? splice @data, $i++, 0, $_:push @data, $_;
}
print @data;

__DATA__
foo 1 zot 
foo 2 blahblah 
bar 1 zot 
bar 2 zot 
bat 1 blahblah 
bat 2 
baz 1 
baz 2 zot
[download]

It works fine and only needs one array.

Cheers,
Ovid

Update: Clearly, I am smoking crack. larsen pointed out that I kept an extra array. Here's what I meant to post:

#!/usr/bin/perl -w
use strict;
my (@data, $i);

for (<DATA>) {
    /blahblah/? splice @data, $i++, 0, $_:push @data, $_;
}
print @data;

__DATA__
foo 1 zot 
foo 2 blahblah 
bar 1 zot 
bar 2 zot 
bat 1 blahblah 
bat 2 
baz 1 
baz 2 zot
[download]

There. Only one array. (sigh)