Idiomatic perl uses split() when you know what you want to throw away and a regular expression match when you know what you want.

Putting a "zero or more characters from this class" regular expression into split (as in split /\n\r\l*/) effectively splits a string into individual characters - the regex is true between every single character. Your code would get further if you tried split /\n\r\l+/ ....

Using regular expression matches, you could do something like:

#!/usr/bin/perl

use strict;
use warnings;

undef $/;         # read whole file at once
our $cert = <>;
our ($info) = $cert =~ m/X509v3 Basic Constraints:\s+([^\n]+)/;
print "Constraints = $info\n";
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