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|