Individual calls to mycmp()...
Same package:      a b => a b
Different package: a b => a b
Same package:      b a => a b
Different package: b a => a b
Same package:      c b => b c
Different package: c b => b c
Same package:      b c => b c
Different package: b c => b c

Multiple calls to mycmp() in sort...
Same package:      a b c => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: a b c => a b c

Same package:      b a c => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: b a c => a b c  <== first two inputs sort correctly

Same package:      c b a => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: c b a => b c a  <== first two inputs sort correctly

Same package:      b c a => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: b c a => b c a