using split can be much easier and cleaner in certain circumstances if it had this functionality

I'm not convinced that an array-based split would be so easy to work with. I think that looping a simple regex match over an array of delimiters would be easiest for the average programmer to grasp (and maintain); something like:

my @delim_seq = qw/: :: \s+ = \n/;
my $string = "a:b::c   d=e\n";
my @fields = ();
foreach my $delim ( @delim_seq ) {
   $string =~ s/(.*?)$delim//;
   push @fields, $1;
}
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(Note that the final delimiter in the sequence is assumed to be the final pattern on the line -- i.e. the line terminator.)

So you have two statements inside a loop, instead of a single statement using "split(...)" -- I could live with that easily enough (whereas I'd worry about adding complexity to a basic function like "split").