$2=ab  # Only thing matched
$2=a   # Only thing matched
$2=a   # First thing matched

##</code><code>##

$2=ab  # Only thing matched
$2=a   # Only thing matched
$2=b   # Last thing matched

##</code><code>##

$ perl -e'"ab" =~ /((\w+)(?{print defined $^N ? "\$^N=$^N\n" : "\$^N not defined\n"})){2}/;'
$^N=ab
$^N not defined
$^N=b