$string = "One, Two, Three"; 
 
# note the capture brackets 
$pattern = qr/,([^,]*)$/; 
 
# single quotes otherwise $x's are interpolated 
$replacement = ' and$1'; 
 
# supports as many matches as you can capture 
@matches = $string =~ /$pattern/;   
 
# defined test prevents warnings about uninitialized values in substitution iterator 
$replacement =~ s/\$(\d+)/$matches[$1 - 1] if defined($matches[$1 - 1])/eg;   
 
# now it works :) 
$string =~ s/$pattern/$replacement/g;