viffer:

How are you displaying it under the debugger? I'm seeing the expected value (using perl v5.10.1 under cygwin):

roboticus@Boink:~$ cat 854238.pl 
my $text = "Kev.has.a.stupid.perl.question";
$text =~ s/\./\\./g;
$text .= ".";
print $text;

roboticus@Boink:~$ perl 854238.pl
Kev\.has\.a\.stupid\.perl\.question.

roboticus@Boink:~$ perl -d 854238.pl
  ...snip...
main::(854238.pl:1):    my $text = "Kev.has.a.stupid.perl.question";
  DB<1> b 3
  DB<2> r
main::(854238.pl:3):    $text .= ".";
  DB<2> p $text
Kev\.has\.a\.stupid\.perl\.question
  DB<3>
[download]

...roboticus

using x $text rather than print $text
ie


my $text = "Kev.has.a.stupid.perl.question";
DB<1> x $text
0  'Kev.has.a.stupid.perl.question'
$text =~ s/\./\\./g;
DB<2> x $text
0  'Kev\\.has\\.a\\.stupid\\.perl\\.question'
[download]

viffer:

Interesting ... I've never noticed that the x command and the p command give different results for a string in the debugger. A quick browse of perldebug didn't give me any obvious hints...

...roboticus