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