$ perl -MO=Deparse -wle 'my ($a) = 1;my $b = 2; (my $c) = 3;my ($d, $e
+) = (4, 5); (my $f, my $g) = (6, 7); (my($h, $i) = (8, 9));my $j; my 
+($k);'
$ perl -MO=Concise -wle 'my ($a) = 1;my $b = 2; (my $c) = 3;my ($d, $e
+) = (4, 5); (my $f, my $g) = (6, 7); (my($h, $i) = (8, 9));my $j; my 
+($k);'
[download]

• my ($a) = 1;
  

  deparse: my($a) = 1;
  
  concise:
  7     <2> aassign[t2] vKS ->8
  -        <1> ex-list lK ->5
  3           <0> pushmark s ->4
  4           <$> const(IV 1) s ->5
  -        <1> ex-list lK ->7
  5           <0> pushmark s ->6
  6           <0> padsv[$a:1,7] lPRM*/LVINTRO ->7
  [download]

• my $b = 2;
  

  deparse: my $b = 2;
  
  concise:
  b     <2> sassign vKS/2 ->c
  9        <$> const(IV 2) s ->a
  a        <0> padsv[$b:2,7] sRM*/LVINTRO ->b
  [download]

• (my $c) = 3;
  

  deparse: my($c) = 3;
  
  concise:
  h     <2> aassign[t5] vKS ->i
  -        <1> ex-list lK ->f
  d           <0> pushmark s ->e
  e           <$> const(IV 3) s ->f
  -        <1> ex-list lK ->h
  f           <0> pushmark s ->g
  g           <0> padsv[$c:3,7] lPRM*/LVINTRO ->h
  [download]

• my ($d, $e) = (4, 5);
  

  deparse: my($d, $e) = (4, 5);
  
  concise:
  p     <2> aassign[t8] vKS ->q
  -        <1> ex-list lKP ->m
  j           <0> pushmark s ->k
  k           <$> const(IV 4) s ->l
  l           <$> const(IV 5) s ->m
  -        <1> ex-list lKPRM*/128 ->p
  m           <0> pushmark sRM*/128 ->n
  n           <0> padsv[$d:4,7] lRM*/LVINTRO ->o
  o           <0> padsv[$e:4,7] lRM*/LVINTRO ->p
  [download]

• (my $f, my $g) = (6, 7);
  

  deparse: my($f, $g) = (6, 7);
  
  consise:
  x     <2> aassign[t11] vKS ->y
  -        <1> ex-list lKP ->u
  r           <0> pushmark s ->s
  s           <$> const(IV 6) s ->t
  t           <$> const(IV 7) s ->u
  -        <1> ex-list lKPRM* ->x
  u           <0> pushmark sRM* ->v
  v           <0> padsv[$f:5,7] lRM*/LVINTRO ->w
  w           <0> padsv[$g:5,7] lRM*/LVINTRO ->x
  [download]

• (my($h, $i) = (8, 9);
  

  deparse: my($h, $i) = (8, 9);
  
  concise:
  15    <2> aassign[t14] vKPS ->16
  -        <1> ex-list lKP ->12
  z           <0> pushmark s ->10
  10          <$> const(IV 8) s ->11
  11          <$> const(IV 9) s ->12
  -        <1> ex-list lKPRM*/128 ->15
  12          <0> pushmark sRM*/128 ->13
  13          <0> padsv[$h:6,7] lRM*/LVINTRO ->14
  14          <0> padsv[$i:6,7] lRM*/LVINTRO ->15
  [download]

• my $j;
  

  deparse: my $j;
  
  concise:
  17    <0> padsv[$j:7,9] vM/LVINTRO ->18
  [download]

• my ($k);
  

  deparse: my $k;
  
  concise:
  19    <0> padsv[$k:8,9] vPM/LVINTRO ->1a
  [download]

Summary:

• my ($a) = 1; and (my $c) = 3; are identical in the OPs it generates. Thus there is no difference in which one you use
• my ($a) = 1; and my $b = 2; are not identical. The generates OPs are different and so is the behaviour (as pointed out already)
• my ($d, $e) = (4, 5); and (my $f, my $g) = (6, 7); and (my($h, $i) = (8, 9)); are identical
• my $j; and my ($k); are identical in the OPs it generates. Thus there is no difference in which one you use

Updated: updated to include my $j; and my ($k);