I would put the escaping backslash into $delim, so you are truly independent. Besides that your code works fine and splits the string as intended.

my ($delim, $line, @fields);

$delim = '\|';
$line  = "a|c||b";
@fields = split(/$delim/, $line);
print join ("*", @fields), "\n";

$delim = ';';
$line  = "a;c;;b";
@fields = split(/$delim/, $line);
print join ("*", @fields), "\n";


#a*c**b
#a*c**b
[download]

What else did you expect?