To expand on imp's suggestion with look-behind assertions, you could use look-behind and look-ahead assertions in combination so that you find a point that is preceded by a pipe symbol or the beginning of the line and followed by a pipe symbol or the newline and do the substitution there. Look-behinds have to be fixed width, hence the alternation of two of them. I have set up a variable $litPipe to hold a literal pipe symbol to avoid a lot of escaping inside the regular expression.

use strict;
use warnings;

my $litPipe = q{\|};
my $rxBetween = qr
   {(?x)
       (?:
           (?<=\A)
           |
           (?<=$litPipe)
       )
       (?=$litPipe|\n)
       
   };

while (<DATA>)
{
    s{$rxBetween}{EMPTY}g;
    print;
}

__END__
a|b|c|d|e
f||h|i|j
|l|m|n|o
||||t
u|v|w|x|
[download]

Here's the output

a|b|c|d|e
f|EMPTY|h|i|j
EMPTY|l|m|n|o
EMPTY|EMPTY|EMPTY|EMPTY|t
u|v|w|x|EMPTY
[download]

Because the regular expression pins down exactly where you want to do the substitution it copes well with beginning and end of line situations.

I hope this is of use.

Cheers,

JohnGG