raygun:

Re-read the perldoc -f split documentation. Since you have a grouping parenthesis in your regex, split is inserting the captured values into the list of values returned. Since '-' is a valid delimiter, and is outside the grouping parenthesis, it's inserting a null string into your list of values.

$ cat 930260.pl
#!/usr/bin/perl
use strict;
use warnings;

while (my $line = <DATA>) {
    chomp $line;

    my @flds = split /-(?=[^-]+(-r[0-9]+)?$)/, $line;
    print join("|", @flds),">\n";
}
__DATA__
mono-basic-2.10
mono-2.10.2-r1
mono-2.10.5

$ perl 930260.pl
Use of uninitialized value $flds[1] in join or string at 930260.pl lin
+e 9, <DATA> line 1.
mono-basic||2.10>
Use of uninitialized value $flds[3] in join or string at 930260.pl lin
+e 9, <DATA> line 2.
mono|-r1|2.10.2||r1>
Use of uninitialized value $flds[1] in join or string at 930260.pl lin
+e 9, <DATA> line 3.
mono||2.10.5>
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...roboticus

When your only tool is a hammer, all problems look like your thumb.

$ cat 930260.pl