I suspect that this question is related to the current Perl Review golfing challenge, but here is a solution to your problem. I could give you a regex-based solution but it would be more complicated and probably less efficient.

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

my @x;
$_="blah";

for my $i (0..length($_)-2)
{
        $x[$i]=substr($_,$i,1).substr($_,$i+1,1);
}

print "@x\n";
[download]

produces...

bl la ah

metadoktor

"The doktor is in."