One way, with regexes, is:

$string =~ s/(..)/$1|/g;
[download]

If the string is read from a handle, there is a neat trick using a reference to an integer for $/:

my $string;
{
    local $/ = \2;
    my @pairs = <FOO>;
    $string = join '|',  @pairs;
}
[download]

It would be good if one of the modules tieing handles to strings honored $/ accurately enough to us this on a string, but afaik they don't.

After Compline,
Zaxo