O wise monks, this is probably one of those questions that has a simple, obvious answer that I'm just not seeing because I've been looking at it too long.

I have the following input: 

mono-basic-2.10
mono-2.10.2-r1
mono-2.10.5
I need to split each line into a package name and a version string. Because the hyphen may occur in either part, I need a regex that is smart enough to figure out which hyphen to split on.

This regex seems to work: 

foreach $line (<STDIN>) {
  chomp $line;
  $line =~ s/-(?=[^-]+(-r[0-9]+)?$)/   ==>   /;
  print "$line\n";
}

[download]
in that it turns the correct hyphen into a more obvious separator, outputting: 
mono-basic   ==>   2.10
mono   ==>   2.10.2-r1
mono   ==>   2.10.5
However, if I use the exact same regex in the split function: 
foreach $line (<STDIN>) {
  chomp $line;
  ($package, $ver) = split /-(?=[^-]+(-r[0-9]+)?$)/, $line;
  print "$package   ==>   $ver\n";
}

[download]
it doesn't work: 
mono-basic   ==>   
mono   ==>   -r1
mono   ==>
Giving split a LIMIT of 2 doesn't change the output; the actual version numbers are eaten, and only the trailing -r1 on the second line makes it into $ver. What is it about split's processing of the regex that is different from that of the substitute operator? 

$ perl --version

This is perl 5, version 12, subversion 3 (v5.12.3) built for i686-linux
(with 13 registered patches, see perl -V for more detail)

In reply to regex behaves differently in split vs substitute? by raygun

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