Running your program under 5.12.2 and 5.14.1 causes some kind of infinite loop in the debugger, so the code doesn't run at all :)

Under 5.6.1, I can access $a and $b no problem

$ C:\MinGW\msys\1.0\bin\perl.exe -d mytest2.pl
Default die handler restored.

Loading DB routines from perl5db.pl version 1.07
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(mytest2.pl:3):   print mt2(55), "\n";
  DB<1> s
mytest2::mt2(mytest2.pm:11):      my $b = $_[0] * 2;
  DB<1> x $a
0  23
  DB<2> x $b
0  undef
  DB<3> n
mytest2::mt2(mytest2.pm:12):      return $a + $b;
  DB<3> x $a
0  23
  DB<4> x $b
0  110
  DB<5> n
133
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.
  DB<5> q
[download]

For future references, your demos should always add up to 42 :p